Seminar Paper

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Power generation through combined cycle mode in steel plants

R. Raghavan, Om Prakash and Dipankar Mondal

TATA CONSULTING ENGINEERS LIMITED

Abstract

The by-product gases from steel plants namely blast furnace gas and coke oven gas are being used as fuels for process steam and power generation. This paper highlights the worldwide experience of utilization of these steel plant off-gases in a gas turbine based power plant to maximize the thermal efficiency. Also, this paper presents the possible options for augmenting power generation in the existing power houses of TATA STEEL in the future scenario of expansion of steel production to 9.7 MTPA capacity.

Key words

Repowering, Combined cycle power plants, Blast furnace gas firing in gas turbines

Introduction

Steel industries generate considerable amount of blast furnace gas (BFG) and coke oven gas (COG) as by-product gases in the steel making process. These off-gases have very low calorific value and have been used as fuels in power plant located within/adjoining steel complex. The need for reliable, efficient and environmentally friendly solutions has driven many industrial users to search for better ways to burn waste gases from industrial plants in conventional thermal power plants. This paper highlights the merits of utilization of BFG & COG in a combined cycle power plant to maximize the thermal efficiency with specific reference to TATA STEEL.

Combined cycle power plant

A combined cycle is a synergistic combination of thermodynamic cycles operating at different temperatures. A gas turbine operates on Brayton cycle. The exhaust gas from the gas turbine can be effectively utilized to generate steam which can drive the steam turbine, which operates on Rankine cycle. Hence, a combined cycle plant achieves maximum thermal efficiency in converting the chemical energy in fuels to electric power. Apart from higher efficiency of energy conversion, the favourable factors for combined cycle plants are lower capital cost, shorter completion period, quick start-up and rapid load response.

Worldwide experience

Most of the combined cycle power plants are based on natural gas firing. However, the leading gas turbine manufacturers namely Alstom, General Electric (GE) & Mitsubishi Heavy Industries Ltd (MHI) have supplied gas turbines suitable for BFG & COG firing. The details of the experience of these manufacturers are furnished below.

Alstom

Alstom has developed the gas turbine model GT11N2 to meet the market demand of about 100 MW capacity with wide range of fuels. The GT11N2 comes with three types of combustor. The EV (environmental burner) dry low NOx combustor is suitable for natural gas. The LBTU combustor can handle fuels such as blast furnace gas with low heating value (LHV) as low as 525 kCal/kg. The single burner (SB) combustor can burn diesel oil and hydrogen synthetic fuels with a hydrogen content of well excess of 50%.

The GT11N2 is one of the biggest gas turbine models in operation worldwide capable of base load operation exclusively burning low calorific value gases without blending with higher calorific value gases. Safe continuous operation has been demonstrated for blast furnace gas with single digit NOx emissions. Its rugged design makes the LBTU combustor the ideal fit for demanding industrial operations.

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In the last 50 years, Alstom has built a significant number of gas turbines designed for applications in steel plants using low calorific value fuels. The operating experience of these machines exceeds two million hours. One of these turbines, installed in 1959 at the Donawitz plant in Austria, has today notched up over a quarter of a million operating hours. Its design is based on that of similar burners installed in more than 20 units operating on blast furnace gas.

The two GT11N2 units in commercial operation at Bao Shan, China and Mizushima, Japan have accumulated nearly 150,000 fired hours.

Bao Steel Corporation of China has installed a GT11N2 unit for operation on LBTU blast furnace gas. Built in cooperation with Kawasaki Heavy Industries, this combined cycle facility aligns gas turbine, gas compressor, steam turbine and electric generator on a single shaft. The gas turbine went into commercial operation in the first quarter of 1997 and has accumulated nearly 60,000 fired hours to date. The plant is designed to provide upto 150 MW of electrical power and upto 180 t/hr of steam to steel plant.

Operation of these two power plants has demonstrated excellent results, confirming the forecast performance and single digit emission levels. With thermal efficiencies over 45 %, such plants are very attractive solution for steel plants burning waste gases.

Alstom is executing a CCPP with GT11N2-2 with LBTU burner for CSA, Brazil. The list of installations supplied by Alstom is furnished in the Table – 1 below.

Table – 1 : Alstom’s installations for CCPPs with BFG & COG firing

Client Plant Year Gas turbine model

Plant output (MW)

Fuel

Bao Shan, China

Cogeneration 1997 1 x GT11N2 150 BFG

Mizushima, Japan

Cogeneration 2002 1 x GT11N2 90 BFG

CSA, Brazil Cogeneration Under construction

GT11N2-2 490 BFG

General Electric (GE)

GE is one of the leading suppliers of gas turbine suitable for low calorific value gases. GE has supplied few gas turbines for steel plants in China & Italy and the details are furnished in the Table – 2 below.

Table – 2 : GE’s installations for CCPPs with BFG & COG firing

Client Plant Year Gas turbine model

Plant output (MW)

Fuel

ILVA ISE Cogeneration 1996 3 x 109 E 520 BFG/COG/ LDG

Piombino Edison

Cogeneration 2001 1 x 109 E 180 BFG/COG/ LDG

Tonghua Cogeneration 2003 6B 50 BFG/COG

Jinan Power 2004 6B 400 BFG/COG

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Baosteel Luojing

Power 2008 1 x 109 E 169 Corex

Wuhan steel Power 2009 2 x 109 E 325 BFG/COG / LDG

The flow diagram for CCPP with GE’s 6B gas turbine is furnished in Figure - 1 below. As can be seen from the flow diagram, the net power output of the CCPP will be about 50 MW i.e. by subtracting the power consumption of fuel gas compressor, which is about 13.3 MW. This configuration is based on multi shaft arrangement with wet electrostatic precipitator for dust removal.

Figure 1 – Flow diagram for CCPP with GE’s 6B gas turbine

The flow diagram for CCPP with GE’s 9E gas turbine is furnished in Figure - 2 below. As can be seen from the flow diagram, the net power output of the CCPP will be about 166 MW i.e. by subtracting the power consumption of fuel gas compressor, which is about 50 MW. This configuration is also based on multi shaft arrangement with wet electrostatic precipitator for dust removal.

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Figure 2 – Flow diagram for CCPP with GE’s 9E gas turbine

Mitsubishi Heavy Industries Ltd (MHI)

MHI has started its gas turbine business in early 1960s under the license from Westinghouse. Later on through constant research and development, MHI has become a leading supplier with the development of 1350 deg. C class M501F/M701F and 1500 deg. C class M501 G / M701 G gas turbines. Also, MHI has developed 1500 deg. C class G series gas turbine that adopts steam cooling technology in the combustor, which is the largest capacity gas turbine in the world with successful commercial operation for both 50 Hz and 60 Hz range.

There are three gas turbine models developed by MHI which are suitable for BFG & COG firing. The typical power output and BFG & COG requirement are furnished in the Table – 3 below. The list of installations supplied by MHI for BFG & COG firing is furnished in the Table – 4.

Table – 3: MHI’s gas turbine models for BFG & COG firing

Gas turbine model

BFG requirement (Nm3/hr)

COG requirement (Nm3/hr)

CCPP typical output (MW)

LHV of mixed gas (kCal/Nm3)

M251 160,000 - 67 740

701DA 242,000 20,000 150 1050

701F 465,000 28,000 300 1050

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Table – 4: MHI’s installations for CCPP with BFG & COG firing

The difference between natural gas fired CCPP vis-à-vis BFG fired CCPP is furnished in Figures - 3a & 3b below. The BFG fired CCPP would consist of a gas compressor coupled to the gas turbine shaft for increasing the gas pressure to meet the gas turbine requirement. Also, in order to reduce the dust content, a wet type electrostatic precipitator would be envisaged.

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Figure 3a – Difference between natural gas fired & BFG fired CCPP (MHI model)

Figure 3b – System Configuration for BFG fired for CCPP (MHI model)

Design modifications carried out by MHI for BFG firing

(I) Gas turbine inlet temperature:

In order to maximize the thermal efficiency, MHI has developed various gas turbine models with higher and higher gas turbine inlet temperature over a period of time and the trend of gas turbine inlet temperature has been furnished in Figure - 4 below.

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Figure 4 – Trend of gas turbine inlet temperature (MHI models)

(II) Technology development due to BFG characteristics

BFG has special characteristics such as low heating value and dust content. The key modifications carried out by MHI in the fuel system, supply system and combustor are summarized in the Table 5 below.

Table – 5: Technology development in gas turbines for BFG firing

BFG characteristics Concern Development

• Narrowed Flammability zone

• Low burning velocity

• To determine the air ratio of fuel nozzle carefully.

• Maintain high burning efficiency

• Combustor section area

• Multi can type combustor with air bypass valve

• Low calorific value • Large capacity gas supply system (gas compressor)

• High efficiency axial flow compressor with variable pitch valve

• Dust content • High efficiency dust removal

• Remove the deposition on gas compressor blades

• Wet type electrostatic precipitator

• Compressor cleaning system

• Toxic • Not to exhaust to atmosphere • Gas cooler & advanced shaft seal

(III) Air compressor:

Higher amount of BFG is to be fed to the gas turbine due to its low heating value. Therefore, if standard gas turbine compressor is retained, the surge problem on air compressor and over load on gas turbine will occur. Hence, in order to maintain the same gas flow to turbine, the air compressor is modified to decrease the air flow by adjusting the height of compressor blades as shown in Figure - 5 below.

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Figure 5 – Air compressor modification for BFG firing (with sectional view of F series gas turbine)

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As an alternative, there is an air bleed system as shown in the Figure - 6 below. In case of air bleed system, the compressor parts are not required to be modified. However, performance is worse than the air flow cut modification during normal BFG firing mode, if there is no possibility to utilize the bleed air.

Figure 6 – Air compressor bleed system for BFG firing

(IV) Combustor

For burning BFG in the gas turbine, the silo type combustor with pilot torch is suitable from the point of view of stable combustion. However, considering the total evaluation of gas turbine including the reliability of turbine blades and other factors, the multi-cannular type combustor is a better option.

The combustor design with this concept is focused from the view point that large amount of the air must be supplied for the combustion because of its substantial low heating value and this gives the disadvantage for the control of fuel to air ratio. Stable and high efficient combustion is required within turn down ratio of 2.5 in the gas turbine combustors.

The disadvantage is only less air is available for the combustor basket cooling. To resolve this concern, multi-cannular combustor design is selected because of the smaller combustor surface area available compared with the large silo type combustor design.

The specially designed variable geometry bypass valve is applied to compensate the air flow supplied to the combustion area. The combustor configuration is shown in Figures 7 & 8 below.

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Figure 7 – Combustor configuration for BFG firing

Figure 8 – Multi-can combustor with air bypass valve

Indian Experience

As on date, there is only one gas turbine plant firing steel plant off gases in India in Neelachal Ispat Nigam Limited, Orissa. BHEL has supplied the gas turbine of 20 MW capacity (Frame 5). The power plant has been provided with a heat recovery steam generator of 45 t/hr capacity. It is reported to have been in satisfactory operating condition.

Possible options for TATA STEEL

The present and future power demand for the steel plant and the source of power supply are furnished in the Table - 6 below.

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Table – 6: Power balance of TATA STEEL plant

Description Present Scenario

(6.8 MTPA)

Future Scenario

(9.7 MTPA)

Power Demand 685 MW 1015 MW

Source of Power supply

In-house generation 183 MW 183 MW

TATA power (Jojobera) 410 MW 525 MW (TATA power & IEL (Jojobera)

DVC 115 307 MW

In order to meet the higher power demand after expansion to 9.7 MTPA, TATA STEEL has to purchase additional power of about 330 MW from external sources. Hence, any power augmentation in the existing power houses will reduce the dependence on external sources.

The BFG & COG consumption, process steam and electric power generation for the present scenario and the future scenario are furnished in Tables 7 & 8 below.

Table – 7: Gas consumption and steam & power generation in TATA STEEL plant (Present Scenario)

Power House BFG

consumption (Nm3/hr)

COG consumption (Nm3/hr)

Process steam supply (t/hr)

Power generation (MW)

Remarks

PH # 3 – (Boilers 5, 6, 7 & 8)

2,40,000 5,000 0 50

PH # 4 (Boilers 1, 2, 3 & 4)

2,53,800 11,000 200 45 Average coal firing rate = 15.6 t/hr

PH # 5 (Boiler A, B & C)

1,88,600 2200 287 20

PH # 5 (Boiler D) 37000 350 40 -

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Table – 8: Gas consumption and steam & power generation in TATA STEEL plant (Future scenario)

Power House BFG consumption (Nm3/hr)

COG consumption (Nm3/hr)

Process steam supply (t/hr)

Power generation (MW)

Remarks

PH # 3 – (Boilers 5, 6, 7 & 8)

110,000 2,000 - 28

PH # 4 (Boilers 1, 2 & 3)

285,000 3,000

200 - 250

45

PH # 4 (Boiler 4) 0 20,000

PH # 5 (Boiler A, B & C)

204,000 1,800 325 25

PH # 5 (Boiler D) 0 0 0 0 Boiler will be stand-by

It is noted that the BFG & COG availability to the PH # 5 in future scenario will be almost same as that of the present scenario. i.e. the process steam and power generation from PH # 5 will be maintained same since PH # 5 is a newly installed unit.

Hence, power augmentation utilizing the gas allotment to PH # 3 and PH # 4 has been studied in this paper.

Repowering of Power House # 3

In future scenario, the gas availability to PH # 3 will be reduced to nearly half and this will cater for only two boilers. The estimated steam and power generation is furnished in Figure - 9 below.

Figure 9 – Flow scheme for PH # 3 (Future scenario)

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In order to increase the power generation in PH # 3, repowering with gas turbine & HRSG option has been studied. In this study, MHI gas turbine (M251S) with single pressure HRSG has been considered. The estimated power generation from the gas turbine with available BFG & COG quantity would be about 22.6 MW. The exhaust heat from the gas turbine would be passed through single pressure HRSG, which will generate steam of about 70 t/hr at 67 kg/cm2 (a) and 488 deg. C. The steam generated would be passed though the existing steam turbine(s). The estimated power generation from steam turbine would be about 15.4 MW. Hence, the total power generation from PH # 3 would be 38 MW (as against the power output of about 28 MW with the present configuration with reduced BFG & COG flow rate).

The preliminary heat and mass balance diagram for the repowering option is furnished in Figure - 10 below.

Figure 10 – Preliminary heat and mass balance diagram for repowering of PH # 3 (Future scenario)

Capital Cost

The indicative / first order capital cost for installation of gas turbine & HRSG and all associated systems would be about Rs. 60 Crores.

Installation of stand-alone CCPP

The PH # 4 is presently supplying about 200 t/hr of process steam and electric power of about 45 MW. If same amount of BFG & COG is utilized in a combined cycle plant, the estimated power output will be about 142 MW. The salient features of the power plant are furnished below.

Gas turbine Model : M701 DA, Single shaft configuration

CCPP power output : 142.5 MW

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BFG requirement : 2,37,400 Nm3/hr

COG requirement : 21,900 Nm3/hr

The preliminary heat and mass balance diagram for the repowering option is furnished in Figure - 11 below.

Figure 11 – Preliminary heat and mass balance diagram for 150 MW CCPP

The typical layout of the 150 MW CCPP are furnished in Figure 12 below.

Figure 12 – Typical layout of 150 MW CCPP (MHI GT model: M701 DA)

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Capital Cost

The indicative / first order capital cost for installation of stand-alone CCPP of about 140 MW with all associated systems would be about Rs. 700 Crores.

Conclusions

The BFG & COG firing in gas turbine is a well established technology and there are several combined cycle power plants in operation worldwide with BFG & COG firing.

Considering the BFG & COG availability for PH # 3 in future scenario (9.7 MTPA), the power augmentation in PH #3 by repowering option will provide an additional power generation of about 10 MW i.e. about 35 % increase of power generation in PH #3.

Considering the BFG & COG availability for PH # 4 in future scenario, the installation of stand-alone CCPP would generate about 142 MW as compared to the power generation of about 45 MW and process steam generation of about 200 t/hr.

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Annexure – 1.0

Analysis of Blast Furnace Gas

Constituents Unit Range (Dry basis) For performance analysis (wet basis)

Carbon Monoxide % Vol. 21 – 25 22.03 Carbon di oxide % vol. 17 – 19 17.24 Hydrocarbon % Vol Nil Nil Hydrogen % Vol. 3.5 – 6.0 4.55 Oxygen % Vol. 0.8 0.78 Nitrogen % Vol. Balance 51.19 H2O % Vol. Nil 4.21 CH4 % Vol. Nil Nil Dust Loading mg/Nm3 50 – 150 50 – 150 GCV kCal/Nm3 800 – 900 804 Pressure at battery limits

mmWC 200 – 300 200 – 300

Temperature at battery limits

Deg. C 30 – 35 30 – 35

Analysis of Coke Oven Gas

Constituents Unit Range (Dry basis) For performance

analysis (Dry basis)

Carbon Monoxide % Vol. 8.8 – 9.0 8.9 Carbon di oxide % vol. 2.8 – 3.2 3.0 Hydrocarbon % Vol 3.0 – 3.4 3.2 Hydrogen % Vol. 51.0 – 53.5 52.3 Oxygen % Vol. 0.8 – 1.0 0.9 Nitrogen % Vol. Balance 9.7 H2O % Vol. Nil Nil CH4 % Vol. 21 – 23 22 Dust Loading mg/Nm3 Nil Nil Tar fog gm/100 Nm3 1.5 – 4.0 1.5 – 4.0 Napthalene gm/100 Nm3 12 – 14 12 - 14 Ammonia gm/100 Nm3 4 – 30 4 - 30 GCV kCal/Nm3 4000 – 4300 4240 Pressure at battery limits

mmWC 350 – 450 350 – 450

Temperature at battery limits

Deg. C 30 – 35 30 – 35

Site data

Ambient temperature: 33 deg. C Relative humidity: 60 %