Boiler

Boilers are vessels that are used to transfer heat produced by the combustion process to a fluid, most commonly steam. The steam or hot water is then pressurized and distributed for applications such as heating and power generation.

Boiler at MNSU [129].

 

Boiler Systems

The main components of the boiler system include the feedwater heaters, deaerators, feed pumps, economizers, superheaters, condenser and condensate pumps. In addition, there are sets of controls to monitor water and steam flow, fuel flow, and chemical composition of the air and steam. The feedwater system is composed of two sources of water that deliver water to the boiler to be converted into steam. The feedwater consists of the condensate return water and treated makeup water. Feedwater heaters extract the waste heat from the spent steam to preheat the boiler feedwater. Preheating the boiler feedwater increases the boiler efficiency by decreasing the energy required to heat the water to steam.

Economizer on boiler #4 at MNSU [129].
Boiler feedwater often contains dissolved oxygen, which comes from air leakages inside the condensers and pumps. The deaerator mechanically removes the oxygen by passing steam through the feedwater heater. The economizer is the last component of the feedwater system. The economizer extracts heat from the exhaust gas to heat the steam to further improve boiler efficiency. Feedwater enters the boiler though the economizer and rises as steam to the steam drum, where the water and steam are separated. The lower drum, called the mud drum, is a tank at the bottom of the boiler that regulates water distribution and collects sediment or corrosion products for later removal.

Water Tube Boilers

Schematic diagram of a water-tube marine-type boiler, drawn using XaraXtreme by Emoscopes 14:26, 10 February 2006 (UTC) [131].

In the circulation process, the non-heated water or pre-heated water enters the feedwater drum from the feedwater pumps. The combustion of fuel in the furnace generates the energy required by the system which is then transferred to the water through a combination of convection and radiation. The combustion gases heat the water into a steam water mixture which, because it becomes less dense than liquid water inside the feedwater drum, rises. The mixture ascends in tubes called risers to the steam drum. In the steam drum, the steam from the water-vapor mixture is removed and released into the system. The water remaining in the steam drum returns to the feedwater drum through pipes called downcomers. The water carried in the downcomers mixes with and pre-heats the water in the feedwater drum.

Boiling water to 100% quality in the tubes is undesirable because water vapor has different heat transfer characteristics than liquid water. This is referred to as low water condition. In low water conditions, the boiler does not receive the amount of feedwater needed to keep it running at safe conditions. Because water acts as a cooling agent inside boilers, low water levels inside the boiler cause the pressure in the boiler to rise. The water absorbs heat created through combustion and turns it into steam. As the amount of water in the steam drum within the boiler decreases, the amount of heat absorbed by the water decreases, thus heating the steam. Because less water is converted to steam, the existing steam is heated and the pressure inside the vessel increases dramatically due to the offset ratio of water and steam. The increased pressure can cause the boiler vessel to explode.

Boiler explosions were quite frequent and extremely dangerous in the late 1800s and early 1900s when boiler technology was still developing, killing hundreds of people.

Boiler explosion at Beaver Mills 1893 [17].
These accidents led to the creation and adoption of industry standards such as the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code . The majority of the boilers in the late 1800s and early 1900s were fire tube boilers which operated like water tube boilers except that the hot combustion gases used to heat the water were passed through tubes submerged in water within the boiler. The boilers exploded because they could not be safely pressurized with the existing technology. This design fault helped launch the development of the modern water tube boiler, which contains the water inside small-diameter tubes where it can reach extremely high pressures with a greater degree of safety.

Minnesota State University, Mankato

Inside Boiler Mud Drum
Inside the boiler mud drum [129].
The boilers used on the Minnesota State University, Mankato, campus are stack drum boilers, in which the drums are stacked one above the other. Also called D type water tube boilers, they are the most common type of small- to medium-sized boilers.

The upper drum, called a steam drum, is where saturated steam leaves the boiler. The lower drum, called the mud drum, receives the liquid feedwater and collects sediments that are carried into the boiler and settle at the bottom of the drum.

The College of New Jersey

The central plant at The College of New Jersey houses two high-pressure steam water tube boilers. These Superior and Cleave-Brooks boilers can be powered with either natural gas or No. 2 fuel oil [62].

On cold days, the steam atomization of oil is used to provide higher combustion efficiencies. Liquid fuels do not burn as readily as gas fuels, so to be able to burn them rapidly, they must be transformed into a vapor-like mist through a process called atomization. The conversion of oil into mist increases the surface area of the fuel and allows it to burn more quickly. The principle of fuel atomization can be compared to burning a log in a fireplace. If you light just one end it will take a long time for the whole log to burn. If you ground the log into sawdust, however, the log will ignite and burn much faster.

Atomizing oils is best achieved using steam. The burner has two supply lines, one for the oil and the other for a jet of steam that assists in the atomization process. The steam delivered to the burner must be dry and of a higher temperature than the oil. The two common problems in the steam atomization process are the loss of heat from the steam due to insufficient insulation around the piping, and the formation of condensation that drips onto the burners due to faulty steam traps.

How Stuff Works → Burners

A burner on a boiler is used to burn fuel efficiently and generate both power for the boiler and steam needed to warm a building. Gas, wood chips, and oil are the most commonly used fuels for the given type of burner.

Boiler Burner
Boiler burner [18].
Each fuel is processed differently: natural gas is sent through a pipe and released into the boiler, wood chips are blown into the burner, and oil is sent through a pressurized tank (like in the oil burner at MNSU), has. Each fuel source is then mixed with air and ignited to create heat. Burners don’t just blow fire into a boiler, however. They must also provide heat and control the outlet pressure of a boiler while burning the fuel as efficiently as possible.

Blowdown

Blowdown is a common procedure used to control contaminants in the boiling water in a boiler. There are two blowdown processes: continuous and manual. In a continuous blowdown, a calibrated valve takes water continuously from the top of the boiling surface in the steam drum, where contaminants, usually oils, are floating. During every shift, workers cool the collected blowdown water in a tank and send it to the city's waste water. The workers then add conditioned city water to the holding tank where the condensate is collected to replace the water removed from the system.

Sootblowing

Inside boiler furnace [129].
Flue gas contains numerous compounds. When combustion occurs some of these compounds will collect and build up on surfaces within the furnace. The buildup will start to affect the overall performance of the boiler by obstructing the heat transfer. To address this problem, two sootblowers are installed inside the furnace: a rotating sootblower and a non-rotating sootblower. Both of the sootblowers extract steam from the steam drum and spray the internal components of the boiler. The rotating blower is attached to a power screw which cleans the steam generating tubes and the non-rotating blower is in a fixed position to clean the duct blower and induced draft fan.

Heat Recovery Steam Generator (HRSG)

Heat Recovery Steam Generators are widely used in power plants and cogeneration/combined cycle systems. In the combined power cycle, hot exhaust gas from a gas turbine is fed into a HRSG to generate steam which in turn drives a steam turbine.

There are four main components in an HRSG: the evaporator, superheater, economizer, and water pre-heater. Within the HRSG are many layers of long tube bundles filled with high-purity water. In a combined cycle, the exhaust gas from the gas turbine passes through the tube bundles which act like a radiator, boiling the water inside the tubes and turning the water into steam. The tube bundles are finned to provide for greater heat transfer. These assemblies are divided into sections. The first section, closest to the output of the gas turbine, is the hottest. The bundles of tubes form racks that divide the HRSG into different sections and which are named for the task they perform. Steam passes through the last component, Economizers (also known as pre-heaters), it exits the stack of the HRSG. The steam generated from the boiling water is distributed among three different pressure steam drums: the high pressure drum (HP), the intermediate pressure drum (IP), and the low pressure drum (LP).

The temperature of the HRSG system must be monitored to accommodate the different thermal properties of the materials. The gas turbine exhaust can become so hot that it bursts the pipe bundles. Automated cooling systems that inject cooling water into the system are installed to prevent pipe ruptures.

Faribault Energy Park

HRSG at Faribault Energy Park
HRSG at Faribault Energy Park [129].
The HSRG at Faribault Energy Park extracts heat from the combustion gases exiting the GE 7FA gas turbine. The hot exhaust gas exits the gas turbine at 1155°F and is sent to the heat recovery steam generator. To ensure high power output, a duct burner is coupled to the HRSG assembly. Duct burners use supplementary firing to increase the heat energy of a gas turbine’s exhaust and the overall energy output. Conventional systems directed the exhaust of the turbine into a burner throat, where fuel was added and mixed with oxygen using high-pressure drops and swirlers, or mixers. Simple grid style systems have been designed to reduce pressure drops and employ an array of fuel manifolds to deliver fuel into the turbine’s exhaust stream. The duct burners are usually used on hot days when the increased temperature at the inlet of the gas turbine which reduces the mass flow rate of the exhaust, or when large amounts of electricity are needed.

Superheaters

Steam tables are used to ensure that the steam running through the system is superheated. A certain amount of superheat in a plant is required in order to avoid saturation in the pipes. When water condenses in the steam pipes, it is very obvious. The condensation causes a loud noise that can be heard throughout the plant and can shift the steam pipes 6 to 7 inches out of place. Superheaters extract all of the liquid water from the steam by raising the temperature of the steam well above the saturation point. The superheater uses the additional energy to heat the steam exiting the boiler. The superheater, which contains horizontally or vertically oriented tubes, is suspended in a convective or radiative zone of the boiler. Having additional heat is important when the steam is delivered to a power generation unit such as a turbine, because excessive moisture in the steam can damage and decrease the efficiency of the turbine. Due to its extremely high temperatures of the steam, high-alloy steels are used to distribute it. The superheater must be constructed with high-alloy steel to handle the extremely high temperatures of the steam.

Economizers

Economizer at MNSU
Economizer at MNSU [129].
An economizer uses the waste heat generated from the combustion process to improve overall efficiency. Flue gas exiting the combustion chamber or gas turbine is still very hot and can be used to pre-heat the feedwater or to transform water into steam to drive a steam turbine (in combined cycle applications).

The economizers at Minnesota State University, Mankato, are horizontal counter-current shell and tube heat exchangers. Feedwater enters finned tubes as hot flue gases pass over the outside of the tube. The energy absorbed by the feedwater would otherwise be lost.

Deaerator

Feedwater for steam generating boilers contains dissolved gases such as oxygen and carbon dioxide. If these gases are not removed, they can line the surfaces in metal piping and other metallic equipment and accelerate corrosion processes. Because these gases cannot be condensed out, a different procedure must be used.

Deaerator at MNSU
Deaerator at MNSU [129].
The types of deaerators commonly used today are spray deaerators and tray deaerators. The former consist of a series of internal cascading trays into which feedwater is directed. Steam bled off from the steam drum is run through a regulator to 10 psig and rises over the trays. When the steam comes in contact with the feedwater a "scrubbing" action takes place, expelling the non-condensable gases from of the water and releasing them into the atmosphere. The most important function of the deaerator is to remove of dissolved gases, but it also can be used to heat and store feedwater, and to prevent feedwater surge.

Unlike conventional power stations, Faribault Energy Park does not contain a deaerator. A pump is used instead of a deaerator to send water to the low pressure HRSG steam drum where it is heated and its oxygen removed.

Fuels

All of the energy required for a boiler is produced through the combustion of a fuel. The burner operates like the gas stove at home, but is more complicated.

Observation Port
Observation port [129].
Burners are comprised of a windbox, air register assembly, igniter, fuel manifold and/or atomizing gun, observation port, and flame safety scanner [124]. Most boilers can burn either No. 2 fuel oil or natural gas. As drivers of flex fuel cars are discovering, being able to choose between fuels can save a lot of money. At any given time, the cheaper of the two fuels can be used in a boiler to minimize the costs of producing steam. The staffs from the utility plant and the state government closely monitor the amount of emissions produced by the boilers by Minnesota State University is allotted a certain quantity of emissions to expel each year. At the end of the year, large institutions must pay per ton of emissions. If they exceed their allotted amount, additional fines are imposed. When the boilers at MNSU were still running on No. 6 fuel oil, which has a high sulfur content , the emissions had to be very closely monitored. The facility was often close to exceeding its sulfur discharge limit.

Combustion

Combustion Flame Inside Boiler
Combustion flame inside boiler [129].
The combustion of fuel in a boiler is a chemical reaction governed by the principles of stoichiometry. The combustive air is provided in excess to guarantee complete combustion. Excess air reduces both the combustion temperature and results in heat loss. The optimum excess air fraction for the combustion process is between 10 and 50% [125]. Incomplete combustion yields toxic carbon monoxide in the exhaust gas.

Sulfur compounds can be found in liquid and solid fuels such as coal and alcohols. Composed of sulfur and oxygen, sulfur oxides, or SOX, are a byproduct of the combustion process. Sulfur dioxide dissolves in water or water vapor to form sulfuric acid, which causes acid rain, so SOX emissions are strictly regulated. High temperatures are maintained in the boiler and stacks to prevent condensation that can cause corrosion.

At the Faribault Energy Park plant, a selective catalytic reduction system (SCR) is used to reduce the amount of emissions from the gas turbine. Installed within the HRSG, the SCR reduces the amounts of nitrogen oxide (NOX) emissions from the turbine exhaust gases by injecting liquid ammonia into the flue gas traveling through the HRSG. The ammonia acts as a catalyst to convert the nitrous oxides into pure nitrogen and water. Using this process, the SCR removes approximately 90% of the nitrous oxides in the turbine exhaust gases [79].

Boilers can produce hot water or steam depending on the needs of the system.
How does a boiler system work? Click to find out.
How can you avoid boiler problems, including low water incident? Read this ASHRAE Journal.
ASME Boiler and Pressure Vessel Code Brochure [123].
MNSU boiler walkthrough video.
How are boilers used in cogeneration? Read this article from ASME.
Want to learn about HRSG design options and benefits? COSPP discusses this here.
Line drawing showing a refrigeration cycle with an economizer.
Feedwater surge is when there is a large and sudden increase in feedwater flow to the boiler.
Have you seen a boiler combustion flame? Watch here.