Performance of Coal Based Thermal Power Plant at Full Load and Part Loads

The present study aims at determining the actual performance of a coal based 210 MW power plant at full and part loads (210 MW and 195.86 MW). Design and actual performance values of boiler efficiency, specific fuel consumption, heat rate and overall efficiency are compared. During actual performance, although small deviations are observed from design values, they may result in substantial loss of revenue and excess operating expenditure. The possible reasons for deterioration are analysed and some possible remedies are suggested.


Introduction
Thermal power plants have played a vital role in improving the power and economic position in India. In every economy, power is a critical input for its development. The quality of life is governed by the per capita power consumption of an economy. For developed countries the per capita power consumption is far too greater than that of the developing countries and hence it is said that the quality of life is very good in developed countries than that in developing countries.
India as a developing economy has added substantially to its installed power capacity in the period right after independence, from 1362 MW in 1947 to 310005 MW on 31 December 2016 [1] (Tables 1  and 2). India, coal found to be the most important and abundant fossil fuel and about 80% of the coal produced consume by India's electricity sector. Also, this is of particular relevance as coal-fired power stations form the backbone of the Indian power generation sector. Mostly, coalfired power plants in India operate on subcritical steam parameters with the exception of a few plants that use supercritical steam parameters. Most of the coal-fired power plants have efficiencies less than 35% by using indigenous high ash coal. Now a day, efforts are taking place to bring in highly efficient super critical technology in the country for thermal power plants. Energy analysis of coal-fired power plant is done using mass and energy balance equation [2].
Coal fired power plants have made a significant contribution in the past in meeting our energy needs and they would continue to play a dominant role in the coming two to three decades. The power industry has witnessed an unprecedented increase in fuel prices and cost of energy production making it imperative to use performance monitoring techniques to effectively control the power plant system for its optimum operating performance [3].

Performance Evaluation Criteria
The power plant performance is measured by assessing the performance of main plant i.e., boiler and turbine-generator as well as auxiliary plants through measurement of prime performance functions like turbine efficiency, boiler efficiency, specific fuel consumption(SFC)  and plant heat rate. To keep high standard of overall performance it is necessary to monitor these performance functions at regular interval. The frequency at which these measurements can be taken, will govern the closeness of optimization control in much the same way as a control system functions to keep a given parameter at its pre-determined value [4,5].

Case Studies
Case study-1: Full load Figure 1 shows the cycle diagram of TPS at operating condition at full load.

Performance Analysis of Thermal Power Station
In order to know the performance of boilers, it is very important to test the efficiency of boilers. Testing of the boiler efficiency calculates the best possible efficiency, which drifts away from the test efficiency. Therefore, corrective action could be taken, to rectify the observed problem areas, for achieving the best efficiency. There are two methods i.e., direct method and indirect method (heat loss method), for the efficiency test of boilers. The heat loss method is more accurate than the direct method. In this method, the efficiency of the boiler is calculated, by subtracting all the heat losses in a boiler from 100 [6].

Dry flue gas loss
This is the heat loss from the boiler in the dry component of flue gases to the stack [4].

Moisture in fuel loss
Compared to bituminous coals, which have less moisture, this reduces the thermal efficiency of the power plant boiler because a considerable amount of the energy from burning the coal is used for in-furnace drying. One opportunity to improve the performance of low-rank coals is to dry the coal using low energy methods such as mechanical thermal expression or steam fluidized bed drying [7]. This is the loss of heat from the boiler in the flue gases due to water vapours which was present initially as moisture in the coal burnt. Moisture that enters the combustion chamber as a part of fuel causes a heat loss [5].

Moisture in combustion loss due to H 2 in coal
Coal contains hydrogen which burns to form water. This loss is due to the heat carried in flue gases by the water which is formed by H 2 [4].

Loss due to unburnt carbon
This loss is due to small amount of carbon which remains as a residue in the ash from boiler. This loss is a function of % ash in fuel and % carbon in ash from boiler. The fineness of pulverized fuel influences this loss [3]. Q uc = m uc ×CV c (4)

Unaccounted losses
Unaccounted losses include moisture in air loss, unburnt gas loss,

Turbine efficiency
Turbine efficiency is the ratio of sum of the actual work done to the sum of isentropic work done of the power plant cycle. Mathematically turbine efficiency is given by: Total actual work done of turbine (Wat) is sum of the actual work done of high pressure, intermediate pressure and low pressure turbines determined on the basis of actual enthalpy drops in different stage and reducing mass flow rates. Total isentropic work done of turbine (Wit) is sum of the isentropic work done of high pressure, intermediate pressure and low pressure turbines [5].

Overall station efficiency
It is the ratio of the electrical energy sent out to the grid to the heat energy of the fuels fired in boiler [5].
Load available at generator terminal coal flow GCV Overall Efficiency = × (6)

Heat rate
Heat rate is the ratio of the heat added to steam in boiler to the electrical energy sent out to the grid. The unit of heat rate kJ/kWh or kcal/kWh. In other words, the heat rate is defined as the heat input to the working substance in boiler required producing one unit of electricity.

Specific fuel consumption (SFC)
It is defined as the amount of fuel consumed per unit of output (power developed) per hour. It is clear indication of the efficiency with which the plant develops capacity (power) from fuel [8].
Fuel consumed in kg / h Power developed in kW SF = (8) Table 3 shows the results which are calculated at the operating conditions of the steam power plant at full load of 210 MW and part load of 195.86 MW.

Discussion and Recommendations
Several technical factors affect the amount of CO 2 emissions from power plants. These include coal type, boiler efficiency, excess air etc. Therefore, for improvements in plant efficiency, it is desirable the operational levels are close to the design levels of parameters. However, that is not the case for several power plants in India. For instance, in a survey carried out by the Indo-German Energy Network, it was found out that almost 25% of the 210 MW units operated at a boiler efficiency of less than 80% and almost 42% of these operated at less than 82% boiler efficiency. Similarly, close to 22% of the 210 MW units operated with an auxiliary consumption of more than 10%. Due to these factors, higher amount of losses occur, which lead to more coal consumption and therefore higher CO 2 emissions. Thus, most old units should be renovated and modernized to achieve higher efficiency [9]. According to losses in boiler it is concluded that dry flue gas loss is within the design range at part load but at full load it deviates. Moisture in fuel loss and H 2 in fuel loss is not in the desired range. Therefore boiler efficiency deviates from the design value. Other losses are little bit near to design value. Unburnt carbon loss also reduces the boiler efficiency.
Comparison between the actual performance and design conditions would be important to evaluate whether the plant is operating according to the design conditions or not. Itemized comparison of various boiler loses is done below. Boiler performance is tabulated below and the deviations are discussed. Summary of results shown in Table 3 is given in Table 4 below.
In the Table 4, design condition of steam generator is recommended by American Society of Mechanical Engineers performance test codes-4 (PTC-4). This Code provides rules and instructions for conducting performance tests of fuel fired steam generators. PTC 4, on Fired Steam Generators, is the ultimate comprehensive document for defining, calculating and testing for the efficiency of Fired Steam Generators by the Energy Balance Method as well as other significant performance parameters. A major feature of the Code is that it includes the methodology for correcting test conditions to guarantee/reference conditions based upon actual unit performance [10].
Actual various losses of boiler at full and part loads are compared with the ASME PTC 4. After that it has concluded whether they are satisfactory or unsatisfactory.

Reasons of higher boiler losses
Though the losses in the boiler are not completely preventable, but they can be minimized but before minimizations of losses, there is need to find the critical area where these losses are maximum so that maximum savings can be done. Some of reasons for higher boiler losses are explored and discussed below.
Dry flue gas loss: Flue gas temperature at the boiler exit is 1550C as against design value of 1400C and incidentally oxygen content at APH outlet is 5%, which is much higher than design value of 3.2% at full load. These factors increase the losses at full load. But these parameters are within the design range at part load. Also the excess air at full load is 31% whereas at part load it is 22%, therefore, loss at full load is more as compared to part load.
Wet stack loss: Wet stack loss is because of moisture and hydrogen in fuel. The total moisture in the coal is 8% as against design value of 6.17% at full load and the total moisture in the coal is 12.5% as against design value of 7.5% at part load. Hydrogen content is slightly deviating from design value. This loss could be reduced by reducing moisture content in the coal as follows.
• Coal must be dried before reaching the combustion chamber so that moisture does not enter the combustion chamber.
• Proper amount of secondary air should be supplied for proper combustion.
• Consistent quality coal by blending the proper mixing to achieve a minimum range of variation in calorific value, volatile matter and moisture.

Unburnt carbon loss:
It is observed that unburnt carbon loss is higher than the design value because of unburnt carbon in bottom ash is 3% higher than the design value at full load. But at part load, unburnt carbon fly ash is 1.2% as against design value of 0.4%. It could be due to poor mill performance and less secondary air for proper combustion due to heavy air leakage from the air heater. This loss should be maximum 1.5% on a better run unit. This loss can be controlled by controlling air supplies and assessing air ingress to mills. This loss can also reduce by proper washing of the coal can reduce the percentage of ash in the coal.