Boiler Basics Design and operation A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat for the steam requirements of process industries or for power generation. Combustion boilers are designed to use the chemical energy in fuel to raise the energy content of water so that it can be used for heating and power applications. Many fossil and non-fossil fuels are fired in boilers, but the most common types of fuel include coal, oil and natural gas.
During the combustion process, oxygen reacts with carbon, hydrogen and other elements in the fuel to produce a flame and hot combustion gases. As these gases are drawn through the boiler, they cool as heat is transferred to water. Eventually the gases flow through a stack and into the atmosphere. As long as fuel and air are both available to continue the combustion process, heat will be generated. Boilers are manufactured in many different sizes and configurations depending on the characteristics of the fuel, the specified heating output, and the required emission controls.
Some boilers are only capable of producing hot water, while others are designed to produce steam. Boilers can burn coal, oil, natural gas, biomass as well as other fuels and fuel combinations. Most boilers are classified as either watertube or firetube boilers, but other designs such as cast iron, coil type, and tubeless (steel shell) boilers are also produced. Components of a boiler system The main components in a boiler system are boiler feedwater heaters, deaerators, feed pump, economiser, superheater, attemperator, steam system, condenser and condensate pump.
In addition, there are sets of controls to monitor water and steam flow, fuel flow, airflow and chemical treatment additions. More broadly speaking, the boiler system comprises a feedwater system, steam system and fuels system. The feedwater system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and repair. The stem system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. Throughout the system, steam pressure is regulated using valves and checked with steam pressure gauges.
The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system. Feedwater system The water supplied to the boiler, which is converted into steam, is called feedwater. The two sources of feedwater are condensate or condensed steam returned from the process and makeup water (treated raw water) which must come from outside the boiler room and plant processes. Feedwater heater Boiler efficiency is improved by the extraction of waste heat from spent steam to preheat the boiler feedwater.
Heaters are shell and tube heat exchangers with the feedwater on the tube side (inside) and steam on the shell side (outside). The heater closest to the boiler receives the hottest steam. The condensed steam is recovered in the heater drains and pumped forward to the heater immediately upstream, where its heat value is combined with that of the steam for that heater. Ultimately the condensate is returned to the condensate storage tank or condenser hotwell. Deaerators Feedwater often has oxygen dissolved in it at objectionable levels, which comes from air in-leakage from the condenser, pump seals, or from the condensate itself.
The oxygen is mechanically removed in a deaerator. Dearators function on the principle that oxygen is decreasingly soluble as the temperature is raised. This is done by passing a stream of steam through the feedwater. Deaerators are generally a combination of spray and tray type. One problem with the control of deaerators is ensuring sufficient temperature difference between the incoming water temperature and the stripping steam. If the temperature is too close, not enough steam will be available to strip the oxygen from the make-up water. Economisers Economisers are the last stage of the feedwater system.
They are designed to extract heat value from exhaust gases to heat the steam still further and improve the efficiency of the boiler. They are simple finned tube heat exchangers. Not all boilers have economizers. Usually they are found only on water tube boilers using fossil fuel as an energy conservation measure. A feedwater economiser reduces steam boiler fuel requirements by transferring heat from the flue gas to incoming feedwaer. By recovering waste heat, an economiser can often reduce fuel requirements by 5 per cent to 10 per cent and pay for itself in less than two years. A feedwater economiser is ppropriate when insufficient heat transfer surface exists within the boiler to remove combustion heat. Boilers that exceed 100 boiler hp, operating at pressures exceeding 75 psig or above, and those that are significantly loaded all year long are excellent candidates for econimiser retrofit. Steam system Steam and mud drums A boiler system consists of a steam drum and a mud drum. The steam drum is the upper drum of a watertube boiler where the separation of water and steam occurs. Feedwater enters the boiler steam drum from the economizers or from the feedwater heater train if there is no economiser.
The colder feedwater helps create the circulation in the boiler. The steam outlet line normally takes off from this drum to a lower drum by a set of riser and downcomer tubes. The lower drum, 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 or corrosion products carried into the boiler. In the circulation process, the colder water, which is outside the heat transfer area, sinks and enters the mud drum.
The water is heated in the heat transfer tubes to form steam. The steam-water mixture is less dense than water and rises in the riser tubes to the steam drum. The steam drum contains internal elements for feedwater entry. , chemical injection, blowdown removal, level control, and steam-water separation. The steam bubbles disengage from the boiler water in the riser tubes and steam flows out from the top of the drum through steam separators. Boiler tubes Boiler tubes are usually fabricated from high-strength carbon steel. The tubes are welded to form a continuous sheet or wall of tubes.
Often more than one bank of tubes is used, with the bank closest to the heat sources providing the greatest share of heat transfer. They will also tend to be the most susceptible to failure due to flow problems or corrosion/ deposition problems. Superheaters The purpose of the superheater is to remove all moisture content from the steam by raising the temperature of the steam above its saturation point. The steam leaving the boiler is saturated, that is, it is in equilibrium with liquid water at the boiler pressure (temperature). The superheater adds energy to the exit steam of the boiler.
It can be a single bank or multiple banks or tubes either in a horizontal or vertical arrangement that is suspended in the convective or radiation zone of the boiler. The added energy raises the temperature and heat content of the steam above saturation point. In the case of turbines, excessive moisture in the steam above saturation point. In the case of turbines, excessive moisture in the steam can adversely affect the efficiency and integrity of the turbine. Super heated steam has a larger specific volume as the amount of superheat increases. This necessitates larger diameter pipelines to carry the same amount of steam.
Due to temperatures, higher alloy steel are used. It is important that the steam is of high purity and low moisture content so that non-volatile substances do not build up in the superheater. Attemperators Attemperation is the primary means for controlling the degree of superheat in a superheated boiler. Attemperation is the process of partially de-superheating steam by the controlled injection of water into the superheated steam flow. The degree of superheat will depend on the steam load and the heat available, given the design of the superheater.
The degree of superheat of the final exiting steam is generally not subject to wide variation because of the design of the downstream processes. In order to achieve the proper control of superheat temperature an attemperator is used. A direct contact attempaerator injects a stream of high purity water into the superheated steam. It is usually located at the exit of the superheater, but may be placed in an intermediate position. Usually, boiler feedwater is sued for attemperation. The water must be free of non-volatile solids to prevent objectionable buildup of solids in the main steam tubes and on turbine blades.
Since attemperator water comes from the boiler feedwater, provision for it has to be made in calculating flows. The calculation is based on heat balance. The total enthalpy (heat content) of the final superheat steam must be the mass weighted sum of the enthalpies of the initial superheat steam and the attemperation water. Condensate systems Although not a part of the boiler per se, condensate is usually returned to the boiler as part of the feedwater. Accordingly, one must take into account the amount and quality of the condensate when calculating boiler treatment parameters.
In a complex steam distribution system there will be several components. These will include heat exchangers, process equipment, flash tanks, and storage tanks. Heat exchangers are the places in the system where steam is used to heat a process or air by indirect contact. Shell and tube exchangers are the usual design, with steam usually on the shell side. The steam enters as superheated or saturated and may leave as superheated, saturated, or as liquid water, depending on the initial steam conditions and the design load of the exchanger.
Process equipment includes turbines whether used for HVAC equipment, air compressors, or turbine pumps. Condensate tanks and pumps are major points for oxygen to enter the condensate system and cause corrosion. These points should be monitored closely for pH and oxygen ingress and proper condensate treatment applied. Fuel system Fuel feed systems play a critical role in the performance of boilers. Their primary functions include transferring 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 the boiler system. Feed system for gaseous fuels Gaseous fuels are relatively easy to transport and handle. Any pressure difference will cause gas to flow, and most gaseous fuels mix easily with air. Because on-site storage of gaseous fuel is typically not feasible, boilers must be connected to a fuel source such as a natural gas pipeline. Flow of gaseous fuels to a boiler can be precisely controlled using a variety of control systems.
These systems generally include automatic valves that meter gas flow through a burner and into the boiler based on steam or hot water demand. The purpose of the burner is to increase the stability of the flame over a wide range of flow rates by creating a favourable condition for fuel ignition and establishing aerodynamic conditions that ensure good mixing between the primary combustion air and the fuel. Burners are the central elements of an effective combustion system. Other elements of their design and application include equipment for fuel preparation and air-fuel distribution as well as a comprehensive system of combustion controls.
Like gaseous fuels, liquid fuels are also relatively easy to transport and handle by using pumps and piping networks that link the boiler to a fuel supply such as a fuel oil storage tank. To promote complete combustion, liquid fuels must be atomized to allow through mixing with combustion air. Atomisation by air, steam, or pressure produces tiny droplets that burn more like gas than liquid. Control of boilers that burns liquid fuels can also be accomplished using a variety of control systems that meter fuel flow.
Feed system for solid fuels Solid fuels are much more difficult to handle than gaseous and liquid fuels. Preparing the fuel for combustion is generally necessary and may involve techniques such as crushing or shredding. Before combustion can occur, the individual fuels particles must be transported from a storage area to the boiler. Mechanical devices such as conveyors, augers, hoppers, slide gates, vibrators, and blowers are often used for this purpose. The method selected depends primarily on the size of the individual fuels particles and the properties and characteristics of the fuel.
Stokers are commonly used to feed solid fuel particles such as crushed coal, TDF, MSW, wood chips, and other forms of biomass into boilers. Mechanical stokers evolved from the hand-fired boiler era and now include sophisticated electromechanical components that respond rapidly to changes in steam demand. The design of these components provides good turndown and fuel-handling capability. In this context, turndown is defined as the ratio of maximum fuel flow to minimum fuel flow. In the case of pulverized coal boilers, which burn very fine particles of coal, the stoker is not used.
Coal in this form can be transported along with the primary combustion air through pipes that are connected to specially designed burners. A burner is defined as a devices or group of devices for the introduction of fuel and air into a furnace at the required velocities, turbulence, and concentration to maintain ignition and combustion of fuel with in the furnace. Burners for gaseous fuels are less complex than those for liquid or solid fuels because mixing of gas and combustion air is relatively simple compared to atomizing liquid fuels or dispersing solid fuel particles.
The ability of a burner to mix combustion air with fuel is a measure of its performance. A good burner mixes well and liberates a maximum amount of heat from the fuel. The best burners are engineered to liberate the maximum amount of heat from the fuel and limit the amount of pollutants such as CO, NOx, and PM that are released. Burners with these capabilities re now used routinely in boilers that must comply with mandated emission limitations.
Conclusion In the past when emission were less regulated, choosing the right boiler and combustion equipment for a particular application generally involved matching the process requirements with the boiler’s output capacity. Proper sizing and selection, peak process requirements and an understanding of the load profile were other key parameters. This boiler selection philosophy emphasized energy conservation at the lowest possible cost. Reduced emphasis was placed on controlling emissions. However, public concern about air and water quality and enactment of federal, state and local regulations have shifted this emphasis.
The current design objective is to provide low-cost energy with an acceptable impact on the environment. Control of PM, NOx, CO, and SO2 emission is now a significant consideration in the overall boiler and combustion equipment design and selection process. Boilers indeed play an important role in process industry operations as well as in power generation. The increasing competition among various boiler manufacturers as well as advancing technology have made this equipment efficient and environment friendly. Reference Book: Power Line Volume 8, No. 3, December 2003