Basics of Steam Boiler


Basics of Steam Boiler

A steam boiler is an enclosed container where water is heated under controlled conditions to convert it into steam. Boiler is basically a heat exchanger where heat is transferred to water. It is also sometimes referred to steam generator. Thermal energy for heating water is supplied either by fuel (Gas, liquid or solid) or by waste energy available from various industrial processes. Sometimes solar energy is also used for the production of steam. Steam produced in a boiler can be low pressure, medium pressure or high pressure.  In an industrial context, the steam produced is used as process steam in various industrial processes or for driving turbines for the production of electricity. Every boiler is designed to transfer as much thermal energy as possible to the water contained in the boiler. Heat energy is transferred by conduction, convection and radiation. The relative percentage of each is dependent upon the type of boiler, the designed heat transfer surface and the fuels that power the combustion.

There are mainly two types of boilers. They are fire tube boiler and water tube boiler.

Fire tube boiler consists of numbers of tubes through which hot gasses are passed. These hot gas tubes are immersed into water, in a closed vessel. In this boiler one closed vessel or shell contains water, through which hot gas tubes are passed. These hot gas tubes heat up the water and convert the water into steam and the steam remains in same vessel.  Fire tube boilers are generally used for relatively small steam capacities and low to medium steam pressures. These boilers are compact, of packaged construction and cheaper.

Water tube boiler is a kind of boiler where the water is heated inside tubes and the hot gasses surround them.  This is just opposite of fire tube boiler. In this boiler, boiler feed water flows through the tubes and enters the boiler drum. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. This boiler is used when the steam demand as well as steam pressure requirements are high as in the case of boiler needed to meet the steam requirements for industrial processes as well as for power generation. The features of water tube boilers include (i) forced, induced and balanced draft provisions helping to improve combustion efficiency (ii) lesser tolerance for water quality hence necessity for water treatment plant, and (iii) higher thermal efficiency levels.

Components of a boiler system

The main components of a boiler system are water treatment plant, boiler feed water heaters, deaerators, feed pump, economizer, superheater, attemperator, steam system, condenser and condensate pump. In addition, there are sets of controls to monitor water and steam flow, fuel flow, air flow and chemical treatment additions.  Schematic of boiler plant flow diagram is shown in Fig 1

boiler plant

                                                                Fig 1 Schematic of boiler plant flow diagram

Broadly a boiler system consists of a feed water system, steam system and fuel system. The feed water system provides treated water to the boiler and regulates it automatically to meet the steam demand. The steam system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment needed depends on the type of the fuel used.

Feed water system

Feed water is the water supplied to the boiler which is converted into steam. The two sources of the feed water are condensate or condensed steam returned from the process and the makeup water which is the treated water from the water treatment plant. The main components of feed water system are as given below.

Feed water heater – Boiler efficiency is improved by the extraction of waste heat from the spent steam to preheat the boiler feed water. Heaters are shell and tube type heat exchangers with the feed water on the tube side and steam on the shell side. The condensate is returned to the condensate storage tank or condensate hot well.

Deaerators – Feed water often has oxygen dissolved in it at objectionable levels, which comes from air in-leakages from the condenser, pumps seals, or from the condensate itself. The oxygen is removed mechanically in a deaerator. Deaerator works on the principle that oxygen is decreasingly soluble as the temperature is raised. This is done by passing steam through the feed water.

Economizers – Economizers are the last stage of the feed water system. They are designed to extract heat value from exhaust gases to heat the steam and improve the efficiency of the boiler. They are simple finned tube heat exchangers. A feed water economizer reduces fuel requirements by transferring heat from the flue gas to the incoming water.

Steam system

Steam system consists of steam and mud drums, boiler tubes, super heaters, attemperators, and condensate systems.

Steam and mud drums – Steam drum is the upper drum of a water tube boiler where the separation of water and steam occurs. Steam drum contains internal elements for feed water entry, chemical injection, blow down removal, level control, and steam water separation. Feed water enters the steam drum from the economizer. Steam flows out from the top of the drum through steam separators. The steam outlet normally takes off from this drum to a lower drum by a set of riser and down comer tubes. The lower drum which is called the mud drum is a tank at the bottom of the boiler that equalizes distribution of water to the generating tubes and collects solids such as salts formed from hardness and silica.

Boiler tubes – Boiler tubes are fabricated from high strength carbon steel. Tubes are welded to form a continuous wall of tube. Usually more than one bank of tubes is used. Tubes are the most susceptible to failure due to flow problems or corrosion deposition problems.

Superheater- Steam when it leaves the boiler is saturated since it is in equilibrium with water at the boiler pressure and temperature. The purpose of superheater is to remove all moisture content from the steam by raising the temperature of the steam above its saturation point. Superheater adds energy to the exit steam of the boiler. The added energy raises the temperature and heat content of the steam above saturation point. Super heated steam has a larger specific volume.

Attemperators – Attemperators control the degree of superheat. Attemperation is the process of partial desuperheating of steam by control injection of water into the superheated steam flow. Usually boiler feed water is used for attemperation.

Condensate systems – Condensate from various heat exchanger systems are returned to the boiler as part of the feed water. However condensates are to be closely monitored for pH and oxygen ingress and proper condensate treatment is to be applied.

Fuel system

Fuel feed systems play a critical role in the performance of boilers. Their primary functions include transfer of the fuel into the boiler and distributing the fuel within the boiler to promote uniform and complete combustion. The type of fuel influences the operational features of a fuel system. The fuel feed system forms the most significant component of a boiler system. Fuels are to be prepared for combustion and transported to the boiler. The combustion system is to ensure stability of flame over a wide range of flow rates by creating a favorable condition for fuel ignition and establishing aero dynamic conditions that ensure good mixing between the primary combustion air and the fuel. Burners are the central elements for an effective combustion system.

Steam boiler efficiency

In steam boiler some energy losses take place which include  incomplete combustion, radiation loss occurring from steam boiler surrounding wall, heat carried by exhaust gases etc.  The efficiency of steam boiler gives indication of these losses. Steam boiler efficiency is the percentage of total heat exported by outlet steam out of the total heat supplied by the fuel as given below.

boiler steam efficiency

Steam boiler efficiency includes thermal efficiency, combustion efficiency & fuel to steam efficiency. Steam boiler efficiency depends upon many factors which include the size of boiler, type of the boiler, design of the boiler etc.

Classification of steam boiler

Based on their design and construction, steam boilers are mainly classified as follow.

Pulverized fuel boiler – Most of the coal fired power station and industrial water tube boilers use pulverized coal. This technology is well developed, and accounts for well over 90 % of coal fired capacity. The coal is ground to a fine powder, so that less than 2 % is +300 microns (?m) and 70-75 % is below 75 microns for a bituminous coal. The pulverized coal is blown with part of the combustion air into the boiler through a series of burner nozzles. Secondary and tertiary air may also be added. Combustion takes place at temperatures from 1300-1700 deg C, depending largely on coal grade. Particle residence time in the boiler is typically 2 to 5 seconds, and the particles must be small enough for complete combustion to have taken place during this time. This system has many advantages such as ability to fire varying quality of coal, quick responses to changes in load, use of high pre heat air temperatures etc.

Fluidized bed combustion boiler – In the fluidized bed combustion (FBC) boiler the firing system is designed based on fluidized bed combustion.  It has significant advantages over conventional firing system and offers multiple benefits such as compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse and other agricultural wastes.

In these boilers, the fluidized bed combustion (FBC) takes place at about 840 to 950 deg C. Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided. The lower combustion temperature is achieved because of high coefficient of heat transfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is maintained between minimum fluidization velocity and particle entrainment velocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream. There are three types of fluidized bed boilers. They are (i) atmospheric fluidized bed combustion (AFBC) boiler, (ii) atmospheric circulating fluidized bed combustion (CFBC) boiler, and (iii) pressurized fluidized bed combustion (PFBC) boiler.

Atmospheric fluidized bed combustion boiler – In this type of boiler coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel fed to the combustion chamber. The atmospheric air, which acts as both the fluidization and combustion air, is delivered at a pressure, after being preheated by the exhaust fuel gases. The in-bed tubes carrying water generally act as the evaporator. The gaseous products of combustion pass over the super heater sections of the boiler flow past the economizer, the dust collectors and the air preheater before being exhausted to atmosphere.

Atmospheric circulating fluidized bed combustion boilers – In a circulating system the bed parameters are so maintained as to promote solids elutriation from the bed. Elutriation is the process in which fine particles are carried out of a fluidized bed due to the fluid flow rate passing through the bed.  Particles are lifted in a relatively dilute phase in a solids riser, and a down-comer with a cyclone provides a return path for the solids. There are no steam generation tubes immersed in the bed. Generation and super heating of steam takes place in the convection section, water walls and at the exit of the riser. CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 – 100 tons/hr of steam.

Pressurized fluidized bed combustion boiler – In this boiler a compressor supplies the forced draft air and the combustor is a pressure vessel. The heat release rate in the bed is proportional to the bed pressure and hence a deep bed is used to extract large amount of heat. This improves the combustion efficiency and sulphur dioxide absorption in the bed. The steam is generated in the two tube bundles, one in the bed and one above it. Hot flue gases drive a power generating gas turbine. The PFBC system can be used for cogeneration (steam and electricity) or combined cycle power generation. The combined cycle operation (gas turbine & steam turbine) improves the overall conversion efficiency by 5 to 8 %.

Waste heat boiler – Wherever the waste heat is available at medium or high temperatures, a waste heat boiler is installed to produce steam economically. The steam may be let down in a steam turbine-generator set and power produced from it. It is widely used in the heat recovery from exhaust gases.