Power plant, River water screening System, Combustion, Gas substation

Introduction:

This is the era of development. Electricity has played a vital role for the socio-economic development of the country. Now almost all the devices are operated by electricity. So to generate electricity, power plant is very necessary.

Power plant:

Power plant is an assembly of equipment that produces and delivers mechanical and electrical energy. A power plant is also referred to as a generating station or powerhouse. It is an industrial facility for the generation of electric power.

River water screening System:

Water is the most important thing in a steam power plant. Water supplies generally are divided into two major categories: surface water and ground water. Surface water is available from rivers lakes and other impoundments as well as the sea. Ground water is present is below the earth’s surface.

In Ashuganj Power plant, water is collected from the river Meghna. This water comes through 3 channels. One channel is for CCPP, ST-1 & ST-2 and one channel is for ST-3, ST-4 & ST-5 and other channel is for demi water. 

This water is used mainly for generation of steam, for condenser, cooling of overall system, cooling of lubricating oil, for chillers etc. So to use the water efficiently water screening system is essential. There are three steps of water screening system. They are described in below:

1.   Heavy-duty Bar: It’s the first stage of screening system. In this stage water is screened by trust rack. Heavy wastes like wooden piece, dead animals, water hyacinth etc. are removed in this stage.

2.      Rotating Bar: In this stage water is screened by scrapers. Smaller waste than heavy-duty bar waste are removed in this stage.

                                 

             

                                  Figure: Water screening system with Rotating and Band screening bar.  

1.      Band Screening Bar: It is a process where 57 baskets are arranged in such a way that screens the water by passing water through net to remove very small wastes like small pieces of stones, sand etc. It is the most effective and modern screening process. A tray is placed below the baskets (net) that takes the waste which is separated from nets by spraying water.

Water Treatment Process:

Water treatment process is very necessary in a steam power plant. In water various solid materials (silt, sand, mud etc), mineral materials (ions like Fe, Ca, K, Mg, Mn etc) and gases (CO₂, CO, O₂, NH₃, H₂ etc) are dissolved. If mineral water is used then it corroded every parts of the plant and forms a layer on glass tubes of boiler. This layer may acts as thermal insulation on the tubes. So when heat is applied then no heat exchange takes place to water from tubes for insulation. Hence the tubes are bent at a high temperature which may blast any time. Finally it causes boiler trip as well as plant trip.

Impurities of water: 

1. Suspended undissolved solid substances (wooden piece, dead animal, sand, silt, dust, stone particles etc.).

2. Dissolved solid and liquid substances (organic and inorganic).

3. Dissolved and free gases (CO₂, Fe₂(SO₄)₃, O₂, H₂O etc).

4. Bacterial substances

Solid impurities removed by filtering but minerals and gases are not removed. So to remove this impurities water treatment process is necessary. 

Working principle:

1. At first the raw water is mixed with Al₂(SO₄)₃ and polyelectrolyte to remove sludge rapidly.

2.  In sedimental basin the sludge are clarified and separated as clean water.

3. Clean water is separated in other basin and the sludge is removed from basin by a pump.

4. Then the water is pumped to the Gravel filter. The “Althracite coal” of the filter removed the solid particles like sand, silt etc.

5.  Finally the water passes through the scavenger, cation, anion and mixed bed filter and removed minerals and gases.

          

         Figure: Water treatment plant

Chemical Reaction:

Cation exchanger:  R-H⁺ +  Na⁺ = R-Na⁺ + H⁺

Anion exchanger:   R-OH^- + Cl^- = R-Cl^-  + OH^-

                                    H⁺ + OH^-  = H₂O

Gas Reaction:           

                         SO₂ + H₂O = H₂SO₃

                                 NH₃ + H₂O = NH₄OH

                         CO₂ + H₂O = H₂CO₃

Cost

The cost for producing 1 liter demi water is 0.11 BDT. In every hour 3.15 tones demi water is supplied to produce 1MW electricity.

Gas substation:

A fuel may be defined as the substance which on burning produce large amount of heat energy. There are many kinds of fuels such as solid fuels (coal), liquid fuels (diesel oil, furnace oil) and gaseous fuel (Natural gas-CH₄). Among them the gaseous fuel is used in APSCL.

Natural Gas Supplied by Titas Gas Transmission & Distribution Co. Ltd., Bangladesh. To control and maintain the gas flow a gas substation is very necessary. Gas is supplied about 30-35 bar. This gas is reduced at a pressure of 4.2 bars in

Valves:

1.      Axial flow gas control valve: It is used to control the axial flow when plant shut down.

2.      Shut off valve: These valves are used to control pressure of the gas.

Combustion:

           

Combustion of any fuel may be defined as a chemical combination of oxygen in the atmospheric air and hydro-carbons. Various kinds of reactions take place which are described in below:

                                    2H₂ + O₂ = 2H₂O

                                    CH₄ + 2O₂ = 2H₂O + CO₂

                                    2CO +O₂ = 2CO

                                    C₂H₄ + 3O₂ = 2CO₂ + 2H₂O