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3. ENERGY PERFORMANCE ASSESSMENT OFCOGENERATION AND TURBINES (GAS, STEAM)

3.1       Introduction

Cogeneration systems can be broadly classified as those using steam turbines, Gas turbines and DG sets. Steam turbine cogeneration systems involve different types of configurations with respect to mode of power generationsuch as extraction, back pressure or a combination of back- pressure, extraction and condensing.

Gas turbines with heat recovery steam generators is another mode of cogeneration. Depending on power and steam load variations in the plant the entire system is dynamic. A per- formance assessment would yield valuable insights into cogeneration system performance and need for further optimisation.

3.2       Purpose of the Performance Test

The purpose of the cogeneration plant performance test is to determine the power output and plant heat rate. In certain cases, the efficiency of individual components like steam turbine is addressed specifically where performance deterioration is suspected. In general, the plant per- formance will be compared with the base line values arrived at for the plant operating condi- tion rather than the design values. The other purpose of the performance test is to show the maintenance accomplishment after a major overhaul. In some cases the purpose of evaluation could even be for a total plant revamp.

3.3       Performance Terms and Definitions

3.4       Reference standards

Modern power station practices by British electricity International (Pergamon Press) ASME PTC 22 - Gas turbineperformance test.

3.5       Field Testing Procedure

The test procedure for each cogeneration plant will be developed individually taking into con- sideration the plant configuration, instrumentation and plant operating conditions. A method is

outlined in the following section for the measurement of heat rate and efficiency of a co-generation plant. This part provides performance-testing procedure for a coal fired steam based co-generation plant, which is common inIndian industries.

3.5.1   Test Duration

The test duration is site specific and in a continuous process industry, 8-hour test data should give reasonably reliable data. In case of an industry with fluctuating electrical/steam load pro- file a set 24-hour data sampling for arepresentative period.

3.5.2   Measurements and Data Collection

The suggested instrumentation (online/ field instruments) for the performance measurement is as under:

Steam flow measurement                   : Orifice flow meters

Fuel flow measurements                    : Volumetric measurements / Mass flow meters

Air flow / Flue gas flow                     : Venturi / Orifice flow meter / Ion gun / Pitot tubes Flue gas Analysis :Zirconium Probe Oxygen analyser

Unburnt Analysis                                : Gravimetric Analysis

Temperature                                       : Thermocouple

Cooling water flow                             : Orifice flow meter / weir /channel flow/ non-contact flowmeters

Pressure                                              : Bourdon Pressure Gauges

Power                                                  : Trivector meter / Energy meter

Condensate                                         : Orifice flow meter

It is essential to ensure that the data is collected during steady state plant running conditions. Among others the following are essential details to be collected for cogeneration plant perfor- mance evaluation.

II.       Electrical Energy:

1. Total power generation for the trial period from individual turbines.
2. Hourly average power generation
3. Quantity of power import from utility ( Grid )*
4. Quantity of power generation from DG sets.*
5. Auxiliaries power consumption

*  Necessary only when overall cogeneration plant adequacy and system optimization / upgra- dation are theobjectives of the study.

3.5.3   Calculations for Steam Turbine Cogeneration System

The process flow diagram for cogeneration plant is shown in figure 3.1. The following calcu- lation procedureshave been provided in this section.

• Turbine cylinder efficiency.
• Overall plant heat rate

Figure 3.1 Process Flow Diagram for Cogeneration Plant

Step 1 :

Calculate the actual heat extraction in turbine at each stage,

Steam Enthalpy at turbine inlet                  : h₁ kCal / kg Steam Enthalpy at 1^stextraction : h₂ kCal / kg Steam Enthalpy at 2^nd extraction                          : h₃kCal / kg Steam Enthalpy at Condenser     : h_4* kCal / kg

*  Due to wetness of steam in the condensing stage, the enthalpy of steam cannot be considered as equivalent tosaturated steam. Typical dryness value is 0.88 – 0.92. This dryness value can be used as first approximation toestimate heat drop in the last stage. However it is suggested to cal- culate the last stage efficiency from the overallturbine efficiency and other stage efficiencies.

Step 2:

Heat extraction from inlet               :       h₁ – h₂ kCal / kg to stage –1 extraction (h₅)

Heat extraction from                       :       h₂ – h₃ kCal / kg 1^st –2^nd extraction (h₆)

Heat extraction from 2nd                :       h₃ – h₄ kCal / kg Extraction – condenser (h₇)

From Mollier diagram (H-S Diagram) estimate the theoretical heat extraction for the conditions mentioned in Step 1.Towards this:

a)   Plot the turbine inlet condition point in the Mollier chart - corresponding to steam pressure andtemperature.

b)   Since expansion in turbine is an adiabatic process, the entropy is constant. Hence draw a vertical line frominlet point (parallel to y-axis) upto the condensing conditions.

c)   Read the enthalpy at points where the extraction and condensing pressure lines meet the vertical linedrawn.

d)   Compute the theoretical heat drop for different stages of expansion.

Theoretical Enthalpy after 1^st extraction                : H₁ Theoretical Enthalpy after 2^nd extraction : H₂ Theoretical Enthalpy at condenserconditions H₃

Theoretical heat extraction from inlet to                : h₁ – H₁ stage 1 extraction, h₈

Theoretical heat extraction from                             : H₁ – H₂ 1^st – 2^nd extraction,h₉

Theoretical heat extraction from                             : H₂ – H₃ 2^nd extraction –condensation, h₁₀

Step 3 :

Calculate plant heat rate*

Heat rate, kCal / kWh                             =

M x (h₁ – h₁₁) P

M – Mass flow rate of steam in kg/hr h₁ – Enthalpy of inletsteam in kCal/kg h₁₁ – Enthalpy of feed water in kCal/kg P –Average Power generated in kW

*Alternatively the following guiding parameter can be utilised

Plant heat consumption = fuel consumed for power generation, kg/hr

Power generated, kW

3.6       Example

3.6.1   Small Cogeneration Plant

A distillery plant having an average production of 40 kilolitres of ethanol is having a cogener- ation system with a backpressure turbine. The plant steam and electrical demand are 5.1 Tons/hr and 100 kW. The process flow diagram is shown in figure 3.2.Gross calorific value of Indian coal is 4000kCal/kg

Figure 3.2 Process Flow Diagram for Small Cogeneration Plant

Calculations :

Step 1 :

Total heat of steam at turbine inlet conditions at 15kg / cm² and 250°C, h₁ =698 kCal/kg

Total heat of steam at turbine outlet conditions at 2 kg/cm² and 130°C, h₂ = 648 kCal/kg

Step 3 :

Heat energy input to turbine per kg of inlet steam (h₁– h₂)         = (698-648) = 50 kCal/kg

Step 4 :

Total steam flow rate, Q₁                                            = 5100 kg/hr

Power generation                                                       = 100 kW

Equivalent thermal energy                                          = 100 x 860 = 86,000 kCal /hr

Step 5 :

Energy input to the turbine                                        = 5100 x 50 = 2,55,000 kCal/hr.

Step 6 :

Energy output

Power generation efficiency of the turbo alternator =--------------------------- 100

Energy Input

86,000

=          ------------- x 100 = 34%

2,55,000

Step 7 :

Efficiency of the turbo alternator                   = 34%

Efficiency of Alternator                                = 92 %

Efficiency of gear transmission                     = 98 %

Step 8 :

Quantity of steam bypassing the turbine            = Nil

Step 9 :

Coal consumption of the boiler                          = 1550 kg/hr.

Step 10:

Overall plant heat rate, kCal/kWh

= Mass flow rate of steam x ((Enthalpy of steam, kCal/kg – Enthalpy of feed water, kCal/kg)

Power output, kW

= 5100 x (698 – 30)

100

= 34068 kCal/kWh*

*Note: The plant heat rate is in the order of 34000 kCal/kWh because of the use of backpres- sure turbine. This value will be around 3000 kcal/kWh while operating on fully condensing mode. However with backpressureturbine, the energy in the steam is not wasted, as it is utilised in the process.

Overall plant fuel rate including boiler              = 1550/100

= 15.5 kg coal / kW

Analysis of Results:

The efficiency of the turbine generator set is as per manufacturer design specification. There is no steam bypassindicating that the power generation potential of process steam is fully utilized. At present the power generation from the process steam completely meets the process electri- cal demand or in other words, the system is balanced.

Remarks: Similar steps can be followed for the evaluation of performance of gas turbine based cogenerationsystem.