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8/13/2019 Effeci-Enger-NARaghu

1/3Chemical Engineering World | 70 | APRIL 2010

Features

Effective Energy Conversion

Cogeneration Opportunities in ProcessIndustries

When we speak of boosting energy efficiency

in industries, Co-generation is the buzzword

being advocated as the most beneficial

technology. Many industries require electricity as

well as heat in the form of steam or hot water for

their operations. These requirements are normallymet through different energy sources. Simultaneous

production of electricity and heat from a single primary

energy source is termed as co-generation or combined

heat and power. Cogeneration is one of the most efficient

energy conversion processes with large cost saving

potential which boosts competitiveness of the industries.

Higher energy conversion efficiencies will also contribute

to lower emission in particular CO2, the main green house

gas. The power generated from cogeneration projects

could be used for meeting the captive requirements and

the surplus power could be exported to the grid.

With the rising energy cost and unreliable grid

power, there is good potential for industries to opt for co-

generation, as a cost reduction measure. As per estimates,

power generation potential through cogeneration is about

15,000 MW through co-generation in core industries of

our country. Significant potential exists in breweries,

chlor-alkali plants, textile mills, distilleries, fertilizer

plants, pulp and paper industry, solvent extraction units,

rice mills, petrochemical plants, etc.

The adoption of cogeneration technology in Indian

industries is poor except in the sugar industries, for

various reasons such as higher capital investment, issues

related to grid synchronization, technological constraints

and fluctuating steam demand etc

A well designed system can tackle all these hurdles.

The cost involved in engineering will get paid back as

Cogen yields good returns apart from undisturbed powersupply.

For better energy efficiency, Steam should be used at

lowest pressure possible. The pressure required in many

process industries are 3 to 4 bar, which achieved using

pressure reducing stations, thus wasting energy. By using

back pressure turbines or extraction cum condensing

turbine, steam at required pressure could be extracted

with out any energy wastage. This will greatly improve

the system efficiency.

Heat-to-power ratio

Heat to power ratio is the ratio of thermal energy to

electricity required by the industry and is expressed in

different units such as Btu/kWh, kcal/kWh, lb./hr/kW,

KW/KW etc. It is the most important parameters to be

considered while selecting the type of cogeneration

system, as it has to match with the characteristics of the

cogeneration system being installed (Refer Table 1)

Designing the downstream system is critical, once we

decide to opt for cogeneration. Designing the steam end

use equipments to operate at the lowest steam pressure

possible would help in extracting maximum power from

Cogeneration is the environmentally-friendly, economically-sensible way to produce power,

simultaneously saving signicant amounts of money as well as reducing greenhouse gas emis-sions. It is a signicant measure to enhance industrial competitiveness

Na Raghu



Features

CogenerationSystem

Heat-to-power ratio

(kWth/ kWe)

Power output(as% of

fuel input)

Overallefficiency

(%)

Back-pressure

steam turbine

4.0-14.3 14-28 84-92

Extraction-condensing

steam turbine

2.0-10.0 22-40 60-80

Gas turbine 1.3-2.0 24-35 70-85

Combined cycle 1.0-1.7 34-40 69-83

Reciprocating engine 1.1-2.5 33-53 75-85

Table 1 Heat-to-power ratios of the different cogeneration systems

the turbine. The table 2 gives an idea of power generationat various back pressures for the same input condition.

To achieve higher turbine efficiency, steam

temperature is important apart from lower back

pressure. Super heated steam helps in better power

output. Table 3 shows the power output for various

steam temperature.

Back pressure turbine

Back pressure turbine would be ideal where the steam

load is steady, without much fluctuation in demand. The

power generation will be to the extent of steam usage.Any extra power demand in this case can be met through

grid power supply. For smaller process boilers this would

be the ideal selection.

Extraction-cum-Condensing Turbines

An extraction cum condensing turbine would be a

better option in the case of fluctuating power and

steam requirements. For many applications in process

industries, this would be the case.

TECHNICALPARAMETERS

BACKPRESSURE 3Kg/cm2 (g)



TurbineInput conditions

10 TPH33 Kg/cm2 g)

10 TPH33 Kg/cm2 g)

10 TPH33 Kg/cm2 g)

ProcessSteam Pressure(Back Pressure)

3 Kg/cm2 (g) 4 Kg/cm2 (g) 5 Kg/cm2 (g)

Power outputfrom turbine

600 KW 540 KW 500 KW

Table 2- Power Generation at Different Back Pressures

Separte production of Electricity and Heat

Total Efficiency :

Cogeneration

Fuel Electricity

100

Fuel

100

Power Plant

Boiler

= 36+80 = 0.5836

Heat

80

200

Fuel

Electricity

100

CogenerationSystem

30

Heat

55

Total Efficiency :

= 30+55 = 0.85100

Fig 2- Efficiency Comparison Between Cogeneration and Separate

Production of Electricity and Head

TECHNICALPARAMETERS

6 TPH9 Kg/cm2(g)

6 TPH21 Kg/cm2(g)

6 TPH21 Kg/cm2(g)

6 TPH33 Kg/cm2(g)

6 TPH42 Kg

/cm2(g)

SteamTemperature 2500

C Saturated 3300

C 3500

C 4200

C

Process SteamKg/cm2 (g)Pressure (Back)

3 Kg/cm2 (g)

3 Kg/cm2 (g)

3 Kg/cm2 (g)

3 Kg/cm2 (g)

3 Kg/cm2 (g)

Power outputfrom turbine

145 KW 220 KW 300 KW 340 KW 380 KW

Quality ofexhaust Steam

Dry -1900C

7 % Wet Dry -2100C

Dry -2100C

Dry -2300C

Table 3 - Power output for different steam temperature

Pel Electrical output

Q heat consumerHQ Heat input

G Generator

feed water

steamshaft

feed waternumn QH

GSteam

generatorSteamturbine

PelQ

Fig 3 - Back Pressure Turbine

Principle

Generator Electrical

Energy

UtilizableHeat

HeatExchanger

Turbine

Steam Cycle

Boiler

Fig 1- Steam Turbine Cogeneration Principle



Features

Rice Mill (par boiling) capacity : 300 ton paddy per day

Steam required for par boiling : 9200 kg / hrInstalled capacity of Boiler : 12000 kg / hr

Installed capacity of turbine : 800KW

Back pressure : 3.5 kg/cm2

Power output during normal operation : 700 KW

Power generated per day : 16,000 units

Power cost : Rs 3.1 / unit

Savings in power generation : Rs51, 000/- per day

The pay back will be around 200 days for the above case.

Case study of a COGEN system in a rice millWith rising cost of fuel, identifying &

implementing small or big cogeneration schemehave become most economic propositions.

Large opportunity exist in Cement, Sugar& Petrochemical plants and Chlor-alkaliindustries ,and cogeneration options need to

be considered/utilized during modernisationand Debottlenecking of existing plants.Detailed steam & power balance could bring

forth this hidden opportunity. Cogenerationcan also help in getting carbon credit andgovernment subsidies.

Na. Raghu, is the CEO of Technoplus Services,

Chennai and is a Boiler, COGEN and Heater

Consultant. He is also a BEE Certifed

Energy Auditor, He can be contacted info@

technoplusservices.com

Process with fuctuating steam loadOften we come across process like breweries, where the

steam demand will be fluctuating. Fluctuating steam

loads hampers boiler operating efficiency. In breweries

the steam pressure used is less than 1 bar, where as the

boiler operates at 9 to 10 bar. The following illustration

explains the steam load pattern with and without using a

steam accumulator.

The system efficiency could be improved by installing

an accumulator. This will help in going for power

generation using cogeneration.

Co-generation BoilersOil, Gas, Coal and agro waste boilers, waste heat recovery

boilers are all used in process industries, which can also

be used for power generation. Since high pressure steam

is required, water tube, smoke tube cum water tube, bi-

drum boilers are used for this application. In India, agro

fuel fired boilers are being used widely to save fuel cost.Manually fired boilers using wood or agro waste are not

suitable for this application, since maintaining stable

steam pressure would be difficult. It is better to consider

Fluidized best combustion (FBC). In case of wood or agro

based residue, a chipper may be installed to facilitate auto

feeding of fuel.

Pel Electrical outputQ heat consumerHQ Heat input

G Generator

feed watersteamshaft

feedwaternumn

QH

GSteam

generatorSteamturbine

PelQ

condenser

Fig 4 - Extraction cum Condensing Turbines

Fig 5 - Co-generation Boilers

Hours

TOTALSTEAMFLOW

HG/HRX1000

NORMAL BOILER

LOAD WITH

ACCUMULATOR

STEMFLOWT

OCOOKE

RS

HG/HRX1000

STEAM CHAROED INTO

ACCUMULATOR

STEAM SUPPLIED FROM

ACCOUMULATOR

6500 KG/HR MEASURED

AVERAGE STEAM LOAD

BOILER LOAD

WITHOUT

ACCUMULATOR

Hours

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