AIR & FLUE GAS PATH

As we have earlier seen the function and working of the boiler. In the boiler combustion takes place and coal’s chemical energy is converted into the heat energy. That heat energy is utilised to heat the water and the water is converted into steam which drive the turbine and ultimately energy is generated.

In this combustion process, in order to burn the coal we require the air. Because for the combustion we require 3 things which are Fuel, Ignition Energy and Oxygen (air). The amount of air in the furnace is also ensuring the complete combustion of coal. After burning of coal, coal is converted into the ash and flue gases. This flue gases contain air as well as light weight ash particles. For proper removal of these flue gases out of furnace we require some amount of pressure or we called it Draft. Because of that draft flue gases evacuate from the furnace. For this we use FD (forced draft) fan and ID (induced draft) fan in the power plant.

Furnace Draft: Furnace draft means to create a pressure difference between the furnace and the atmosphere (up to Chimney) to maintain the flow of flue gas in the furnace. As the the flue gas has a tendency to move upward due to buoyancy, which happens in the 1^st Pass of the Boiler, but in the second pass also we need the flue gas to flow, which is just the reverse direction of flow as in case of 1^st Pass.

This is the main reason why we are creating a draft in the furnace and after creating draft in the furnace, we also need to maintain it.Because there may be chance of Furnace explosion if the pressure inside the furnace is high. Generally we maintain a furnace draft or a furnace pressure of around – 10 MMWC (mm of water column), which is below the atmospheric pressure.

As we have discussed earlier that, air is the main component for the burning of fuel (coal). So for this, FD fan provides air to furnace for proper combustion of coal. The amount of air required for the complete combustion is called “Schiochemetric Ratio” with respect to the fuel input. But in actual practice apart from the air, we have to give some amount of excess air for ensuring that the whole surface area of the coal gets the proper amount of air.

If we give lesser amount of air for combustion then there is a chance of incomplete combustion which results in unburnt coal particles in bottom ash hopper and generation of CO instead of CO2.Similarly if we give excess amount of air then there is a chance of heat loss. Because of that both the condition is avoided with these coal and air,

 3 T’s also required for proper combustion these are –

• Time
• Temperature
• Turbulence

Here the temperature is required for combustion of fuel in furnace to provide sufficient amount of ignition temperature to coal and to convert water into steam. Sufficient time is also required for burning of coal so less unburnt particles are present and complete combustion takes place. Apart from this we also require turbulence for proper mixing of coal with the air.

Secondary Air Flow

After the FD fan, the secondary air goes to the APH (air preheater), this air preheater as the name signifies it heats the air before going to the boiler furnace which increases the combustion efficiency of boiler and another perspective is that lesser fuel is required for the burning which contribute in the productivity of the plant and this also increase the steam generation capacity. In some plants,SCAPH(steam coil air preheater ) is provided to preheat the air before going to air preheater, this prevent the cold end corrosion and increase the life of air preheater during Start-up condition. When the atmospheric air is passing through the lower end side of air preheater then there is a chance of corrosion of lower end of APH (air preheater), this we call Cold End Corrosion.

After passing the APH, the air goes directly to the wind box. In PF (Pulverised Fire) boiler there are 2 numbers of wind boxes. Through these wind boxes air enters into the furnace. We also maintain the air between the wind box and furnace for proper flow of air into the furnace. We maintain slightly negative draft in the furnace to avoid flame outgoing from the furnace for this reason we start firstly ID fan and then FD fan for balancing draft. Normally in power plant the ID fan is of bigger capacity than the FD fan because ID fan sustain hot flue gases and it is also responsible for creating a negative pressure in the furnace with the help of FD fan. The flue gas generated in the furnace, its velocity also should be optimum. If the velocity of the flue gases is much more than there is chance of erosion of the material in the boiler and if flue gas velocity is too low than there is no proper evacuation of flue gases from the furnace.

Apart from ID and FD fan we also use Scanner Air Fan, Seal Air Fan, and PA (Primary Air) Fan in power plants. Here Seal Air Fan is used for sealing of bearings from coal dust in coal mill and coal feeder and increase the life of mill and feeder. Scanner Air Fan is used for cooling of flame scanners which are placed in the furnace for monitoring the flame. Ignitor air fan is used for cooling of ignitor which is used to give ignition energy for burning of fuel and another one is PA fan which is used for coal conveying from coal mill to boiler furnace and for drying purpose of coal in the mill.

(Forced Draft Fan)

(Primary Air Fan)

(Induced Draft Fan)

Primary Air System

For PA fan 2 types of systems are used which are

1. Cold PA system
2. Hot PA system

For drying purpose of the coal we require some amount of hot air. For this purpose PA air stream is divided into two parts which are Cold Primary Air and Hot Primary Air. Cold Primary Air is the air which is directly coming from the atmosphere and Hot Primary Air is taken after APH. Before the coal mill we mix both Cold PA and Hot PA and this optimum temperature air is used for drying purpose of coal, which is around 80°C. PA system also varies from design to design.

In Cold PA system, PA fan takes air suction from atmosphere and some amount of this air goes to APH(Air Pre Heater) and other half of the air directly goes to a header called Cold PA Header. After the APH(Air Pre Heater), Hot air is sent to a Header called Hot PA Header; from there two headers separate taping are taken for mixing of Hot and Cold Primary Air for each Coal Mill and before the mill they are mixed to achieve the desired temperature, which is 80 degree Celsius. Where in case of Hot PA System, PA fans don’t take suction from atmosphere but it takes suction from two separate headers; Hot Primary Air Header and Cold Primary Air Header. Hot Primary Air header receives Air from the APH outlet of secondary Air and Cold Primary Air Header receives air from the FD Fan discharge, before the APH. Both systems have their own advantages and disadvantages.

(Cold Primary Air Cycle)

(Hot Primary Air Cycle)

Generally in BHEL design we use Hot PA system in which all coal mills will have their separate PA fans. But here the PA fan sustains more hot air so the maintenance is a bit difficult.  In Chinese designs we use Cold PA system, in which 1 PA fan for more than 1 mill, here the maintenance of fan is easy but if one PA fan trip it will trip more than 1 mill which is the disadvantage of cold PA system.

Flue Gas Flow

After burning of coal it is converted into Fly Ash and Bottom Ash. This Fly Ash contain huge amount of heat energy. So we utilise that energy to increase the efficiency of boiler. In flue gas path, we place boiler auxiliaries which increase the efficiency of boiler by absorbing the heat content in the Flue Gas. Generally these flue gases pass through Super Heaters and Re Heaters. The number and nomenclature of Super Heaters and Re-Heaters change according to the plants and designs. Generally in two pass boiler Gooseneck area provided in the boiler to give path for flue gases to the next pass. Before the Gooseneck area, heat transfer is through radiation, so we call this area, Radiation Zone and after the gooseneck area, heat transfer is through Convection Heat transfer. In some boiler there is   burner tilting arrangement , if we up the angle of burner it increase the radiation heat transfer and if we down the angle it increase convection heat transfer. After passing through super heater and re heater these flue gases pass through the economiser and then air preheater. After these consecutive heat transfer zones the temperature of the flue gas goes down. After passing through APH flue gases pass through ESP (Electro Static Precipitator). ESP is nothing but dust /fly ash removal system. After passing through ESP flue gas goes to ID (Induced Draft) fan. ID (Induced Draft) fan suck that flue gas and throw it to the Chimney. In flue gas the amount of oxygen is also important in respect of leakage. Because of that we placed an Oxygen Analyser in flue gas path generally before the APH. If the oxygen amount is not in permissible limit then there is leakage in that system. In flue gases sometime SOx and NOx formation is there, because of Nitrogen in air and Sulphur present in coal. This NOx formation is due to very high temperature in furnace. To reduce this we use Over Fire Elevation to cool the flue gas or Ammonia injection in flue gases. Similarly if the flue gas temperature is below its dew point temperature there is chance of formation of sulphuric acid to avoid that we maintain the flue gas temperature above its dew point temperature. Or where the Sulphur content is more in the coal, in that design we use flue gas Desulphurization plant.This is an overview of Air & Flue gas system,Later we will be discussing on the Fans details.