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U.P.B. Sci. Bull. Series D, Vol. 74, Iss. 1, 2012 ISSN 1454-2358 COMBINED HEAT & POWER INDUSTRIAL PLANTS WITH GAS TURBINES & HEAT RECOVERY STEAM GENERATORS – FEASIBILITY STUDY METHOD AND CASE

ANALYSIS

Diana TU?IC?1, Victor CENU??2, Florin ALEXE3

The Combined Heat & Power Industrial Plants (CHPIP) using Backpressure Steam Turbines (BST CHPIP) have low ratios power vs. heat. Therefore is difficult

for them reaching the norms for “high efficiency cogeneration” (HEC). That’s why

the paper analyzes the CHPIP with Gas Turbines & Heat Recovery Steam

Generators (GT&HRSG CHPIP). The main paper’s objectives are, from the

theoretical point of view: establishing an accurate methodology, giving advices for

choosing the main equipments and making the technical section of feasibility studies

on GT&HRSG CHPIP. From the practical sight: applying the methodology in a

case study for a CHPIP, in order to establish if this one could reach the Romanian

HEC norms.

Keywords: CHP, high efficiency.

1. Introduction

The main features of the Combined Heat & Power Industrial Plants (CHPIP) are [1]:

Have flat load curves, with small differences between winter and summer.

Should constantly supply the heat consumer; having good availability. Any supply disruption could generate important damages.

Usually deliver superheated steam, its parameters being imposed by the typical processes. In heating processes, the imposed parameter is the steam’s pressure; it determines the liquefying temperature. This one should be higher than the desired temperature in the process. To avoid the condensing in transport pipes, the delivered steam should have a temperature with about 15 to 25 degrees Celsius higher than the saturation one. If the steam feeds turbines, which turn process compressors, it must have higher pressures and superheating temperatures.

Even if they have high global efficiencies, (electricity + heat) vs. fuel’s primary energy, the CHPIP using Backpressure Steam Turbines (BST CHPIP) have, because of high thermal level of the delivered steam, low ratios power vs. heat.

1 Eng.PhD Student, Power Engineering Faculty, University POLITEHNICA of Bucharest, Romania

2 Lecturer, Power Engineering Faculty, University POLITEHNICA of Bucharest, Romania

3 Prof., Power Engineering Faculty, University POLITEHNICA of Bucharest, Romania

4 Diana Tu?ic?, Victor Cenu??, Florin Alexe

Therefore is difficult for them reaching the norms for “high efficiency cogeneration4” (HEC) [1, 2, 3, 4].

That’s why the paper analyzes the CHPIP with Gas Turbines & Heat Recovery Steam Generators (GT&HRSG CHPIP). The main paper’s objectives are:

A)From the theoretical point of view: establishing an accurate methodology, and

giving advices for choosing the main equipments and making the technical section of feasibility studies on GT&HRSG CHPIP.

B)From the practical sight: applying the methodology in a case study for a

CHPIP delivering an important quota of High Pressure (HP) superheated steam for process turbines, in order to establish if this one could reach the Romanian HEC norms.

2. Methodology; main input data and options for the case study:

The main steps of a technical branch of the feasibility studies on GT&HRSG CHPIP are:

2.1.Establishing, together with the study’s beneficiary, a realistic heat demand

prognosis, and generating the associated load curve.

For the case study we selected a chemical factory requiring steam at two levels: Low Pressure (LP), at 10 bar abs / 195°C, and HP, at 64 bar abs / 450°C. The thermal load curves have two zones: winter and summer, and for each of them the load was considered having a crop Gaussian distribution, characterized by average values and standard deviations. Fig. 1 shows the yearly heat load curve.

2.2.Selecting the appropriate GT number and their type.

Referring to the GT number, we mention that:

A single unit GT&HRSG CHPIP reduces the investment’s costs, but

lessens the availability and reliability, requiring quickly steam supply backup equipment.

If a medium time steam supply disruption could generate important damages, it is proper to use two GT (2*50 %).

Referring to the GT data, we mention that, in order to attain a high power vs. heat ratios, and a good GT&HRSG CHPIP global efficiency, it is suitable to choose GT:

from the thermodynamically point of view, having, in the same time, an elevated electric efficiency (ηel gen cl) and high flue gases exit temperature (t ex gas) [5, 6];

related to their size, allowing to recover an amount of thermal flow rate equivalent to the average summer need.

4 That signifies that a CHP realizes a required fuel economy comparatively with the separate generation. The Romanian norms offer a “bonus” for each electricity unit produced in HEC, delivered in the National Grid, and sold on the competitive market, if the comparative fuel economy is higher than 10 %.

Combined heat & power industrial plants with gas turbines & heat recovery steam generators (5)

304050607080901001101201301400366732109814641830219625622928329436604026439247585124549058566222658869547320768680528418

Time, hours per year

T h e r m a l f l o w r a t e s , M W

HP+LP steam demand HP+LP steam from recovery HP steam

demand LP steam demand

Fig. 1 Thermal load curve for the case study

In our case study we selected 2x SGT-800 GT, with ηel gen cl =37.55%, and t ex gas =544.5°C. In fig.1 we show the amount of thermal flow rate recovered from the two GT at ISO conditions, computed with Gate Cycle soft [6].

2.3. Choosing the HRSG design and the GT&HRSG CHPIP general schedule

(see Fig. 2). For the aforesaid reasons we selected a two pressures HRSG design. For covering the winter heat demands, higher than the recoverable heat from GT exhaust gases, we provided the HRSG with a Duct Burner (DB), using exit flue gases as oxidant.

2.4. Modeling the GT&HRSG behavior at various thermal delivered flow rates,

off design conditions, within the yearly load curve range, and the associated generated power. The winter loads, and some of the summer ones, are with the Combustion Chamber Burner (CCB) at full load, provide the nominal electrical load, and, for achieving the required thermal flow rate (bigger than the recoverable one), use the duct gas burner. The summer partial loads use the CCB at part load, provide a smaller power flow rate, and do not use the DB.

2.5. Establishing the curves describing the yearly evolution of generators clams

output, and thermal flow rates associated to fuel’s burning, for its Low Heat Value (LHV).

2.6. Computing the yearly energy input and output flows and the performances

indicators for the entire GT&HRSG CHPIP.

6 Diana Tu ?ic ?, Victor Cenu ??, Florin Alexe

3. Results:

Fig. 3 & 4 illustrate the GT&HRSG CHPIP behavior, for the case study, at winter and summer thermal loads bigger that the recoverable ones, using the DB. They put into evidence that for each 1 MW th required steam heat flow rate increase, it is necessary to add around 1 MW th heat flow rate increase at DB.

048121620242832

8121620242832Steam heat flow rate increase, MW D u c t b u r n e r f u e l f l o w r a t e , M W

0481216202428320

8121620242832Steam heat flow rate increase, MW D u c t b u r n e r f u e l f l o w r a t e , M W

Fig. 3 Duct burner fuel flow rate vs. steam flow rate increase (winter) Fig. 4 Duct burner fuel flow rate vs. steam

flow rate increase (summer)

Fig. 5 & 6 describe the temperature profile in HRSG without and with DB in use.

Their comparison is relevant, and do not require special comments.

Combined heat & power industrial plants with gas turbines & heat recovery steam generators (7)

without DB in use with DV in use

In summer thermal part loads, under the recoverable heat flow rate, for maintaining the global efficiency, the thermal fuel CCB flow rate should diminish. Fig. 7 & 8 explain the GT&HRSG CHPIP behavior, in the case study, at summer thermal part loads.

Fig. 7 shows that for each 1 MW th required steam heat flow lessen, under the recoverable one, it is necessary to decline the CCB fuel’s flow thermal rate with about 2.64 MW th .

Fig. 8 shows that 1 MW th CCB fuel’s flow thermal rate diminish, corresponds to a generators clams reduction with about 0.432 MW el .

-22

-20-18-16-14-12-10-8-6-4-20-9-8-7-6-5-4-3-2-10

Steam heat flow rate decrease, MW C C f u e l f l o w r a t e d e c r e a s e , M W

-10

-9-8-7-6-5-4-3-2

-10

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0CC fuel flow rate decrease, MW P o w e r o u p u t d e c r e a s e , M W

Fig. 7 CC fuel flow rate decrease vs. steam

flow rate decrease (summer)

Fig. 8 Power output decrease vs. CC fuel

flow rate decrease (summer)

As a result of these, the generators outputs in these off design conditions are smaller than the nominal one. On the other hand, in winter off design conditions the generators clams power output will be higher than the nominal one. As a result, we obtained the electrical load curve from Fig. 9.

8 Diana Tu ?ic ?, Victor Cenu ??, Florin Alexe

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366

732

1098

1464

1830

2196

2562

2928

3294

3660

4026

4392

4758

5124

5490

5856

6222

6588

6954

7320

7686

8052

8418

Time, hours per year

G e n e r a t o r s c l a m s o u t p u t , M W

Fig. 9 The power load curve resulted in the case study

The curves from Fig. 10 describe the yearly evolution of thermal flow rates associated to fuel burning (LHV), and steam flow rates from recovery, respectively from duct burning, in the case study. Numerically integrating the obtained results, we computed: the energy amounts, the efficiencies, and the power vs. heat ratios, without and with DB (see Table 1). With these data we build the diagrams shown in Fig. 11 & Fig. 12, putting in the evidence the average energy flow rates, and the energy quotas, per seasons and yearly.

1020304050607080901001101201301401501601701801902002102202302402502602702800366732109814641830219625622928329436604026439247585124549058566222658869547320768680528418

Time, hours per year

T h e r m a l f l o w r a t e s , M W rate

flow rate

from recov ery

from after burning

Duct burner fuel heat flow rate

Fig. 10 The yearly evolution of thermal flow rates associated to fuel burning (LHV), and steam

flow rates from recovery and from duct burning

Combined heat & power industrial plants with gas turbines & heat recovery steam generators (9)

Table 1

The energy amounts, the efficiencies and the power vs. heat ratios for the analyzed CHPIP,

without and with DB

No Computed data Units

Values

winter summer yearly

1 Heat produced by fuel at DB (LHV) MWh th 64356964674 003

2 Heat produced by fuel at CCB (LHV)MWh th 10975618864511984 012

3 Total heat generated by fuel (LHV) MWh th 11590328931732052

205 4 Electricity at generators clams MWh el 417141323058740 199 5 HP steam heat consumption MWh th 438093347277785 370 6 LP steam heat consumption MWh th 132088104615236 703 7 Total steam heat cons. = prod. MWh th 5701814518921022 073 8 Gas turbines electrical efficiency %38.0136.4437.31 9 GT global efficiency, without DB %87.20187.03887.112

10 Power vs. heat ratio, without DB kJ el /kJh th 0.7725680.7203220.749105 11 CHPIP global efficiency, with DB % 85.1986.7685.87 12 Power vs. heat ratio, with DB kJ el /kJh th 73.1671.4972.42

255075100125150175200225250275 Fuel,average winter

Fuel, yearly average

Fuel,average summer Output,average winter Output,yearly average Output,average summer

E n e r g y a v e r a g e f l o w r a t e s , M W

burner

Fig. 11 The average energy flow rates in the case study, per seasons and yearly

102030405060708090100 Fuel,average winter

Fuel, yearly average

Fuel,average summer Output,average winter

Output,yearly average Output,average summer

E n e r g y q u o t a s , %

burner burner

Fig. 12 The average energy quotas in the case study, per seasons and yearly

The obtained results led us to the indicators shown in the Table 2, where we can see the quotas of fuel energy converted in electricity, respectively in heat. These data were put in the formula required by Romanian norms, in order to determine the fuel economy obtained by CHP generation, comparatively with the separate one. In the last row of Table 2 we can observe that the analyzed CHPIP realizes a comparative fuel economy higher than 19 % all over the year, taking or

10 Diana Tu?ic?, Victor Cenu??, Florin Alexe

not into consideration the duct burner contribution. Consequently it could be classified as a “high efficiency cogeneration plant”, and it may receive the bonus for all the produced electricity.

Table 2 Calculated indicators for the CHPIP case study, per seasons and yearly, without and with

the duct burner’s contribution

Calculated indicators

Without DB With DB Winte

Summe

Yearl

Winte

Summe

Yearl

Share of fuel energy converted in electricity,

38.0136.4437.3135.9936.17 36.07 Share of fuel energy converted in heat, % 49.1950.5949.8049.1950.59 49.80 Comparative fuel economy, % 21.5520.6521.1419.0920.32 19.63

4.Conclusions:

On the market there is a large variety of CHP technologies and equipments, having different manufacturers and thermodynamic designs. The authors selected to analyze the CHPIP, because, due to the previous mentioned features, is more difficult for them satisfying the norms for HEC. The technical solution is choosing the GT&HRSG CHPIP proper technologies and equipments.

The paper is organized in two connected steps. The first one is principally methodological, but, in the same time, identifies the main input data for the case study, and gives advices for equipments and design options. The succeeding step consists in the case study and the graphically and numerically analysis of the obtained outcomes.

The results are useful for power utilities, respectively for power & heat auto producer’s staffs, facilitating them to choose the proper CHP technologies and equipments, and predicting the technical GT&HRSG CHPIP energy flows balance, for improving the cost-effective flows, by benefiting from the HEC bonus.

R E F E R E N C E S

[1]. V. Athanasovici, ?.a. - Tratat de inginerie termic? - Aliment?ri cu c?ldur?, Cogenerare, Editura

AGIR, Bucure?ti, 2010, ISBN 978-973-720-314-4

[2] , Fl. Alexe, , V. Cenu??, - “Economia de energie ?n cogenerare – dependen?a de randament,

indice de termoficare ?i combustibil”, a IX-a Conferin?? Na?ional? de Echipament Termomecanic Clasic ?i Nuclear ?i Energetic? Urban? & Rural?, Bucure?ti, pag 1-4, ISSN 1843-3359

[3]. * * * - Metodologie de calcul pentru stabilirea cantit??ilor de energie electric? produse ?n

cogenerare de eficien?? ?nalt?, ?n vederea certific?rii prin garan?ii de origine,Monitorul Oficial, Partea I nr. 831 din 03/12/2009.

[4]. * * * - Ordin nr. 13/2007 din 22/06/2007, privind valorile de referin?? armonizate aplicabile la

nivel na?ional ale eficien?ei pentru producerea separat? de energie electric?, respectiv de energie termic?, ?i pentru aprobarea factorilor de corec?ie aplicabili la nivel na?ional, Monitorul Oficial, Partea I nr. 434 din 28/06/2007

[5]. J. H. Horlock, , - “Advanced Gas Turbine Cycles”, Whittle Laboratory Cambridge, U.K., 2003

[6]. * * * - Gate Cycle 60.04 GE Software Product