How Boilers Are Used in Power Generation: The Combustion System
March 21, 2014
Industrial boilers are an integral part of the U.S. power generation system. They are literally responsible for keeping the lights on for millions of Americans all across the nation. Boilers accomplish this crucial task indirectly by way of converting the stored energy found in a fuel source into thermodynamic energy (in the form of steam). The thermodynamic energy is then used to power the turbines that ultimately create the consumable power.
What Is Combustion and Why Is It Necessary for Boilers?
In order for boilers to create the thermodynamic energy found in steam they must release energy from a fuel source. This is done using combustion. Combustion is the process of igniting and burning a fuel source. As the fuel source burns it releases energy in the form of heat which can then be used to transform water into steam. Combustion is needed to power the boilers.
The combustion system of the boiler and how efficiently it works, is at the core of operating an efficient boiler. Many significant advances in boiler technology are a direct result of optimizing the combustion system and allowing the boiler to be more fuel efficient. The more fuel efficient the boiler, the lower the cost of operation and the better the energy output.
Types of Boiler Fuels
Boilers can be powered using a wide variety of different combustible fuel sources. When designing a boiler’s combustion system, consideration is usually given to the cost, availability, and efficiency of a given fuel source. The burning characteristics of the fuel source are also important, including factors such as how hot it will burn, smoke and ash output, and environmental concerns. Boilers using renewable sources of energy for combustion are also growing in popularity. Some common boiler fuel sources include:
Coal - Coal-fired boilers are the most popular type of industrial boiler. Coal has been the fuel source of choice for boilers over most of their history and it remains the most pervasive, particularly in the power generation industry. Coal represents an abundant, relatively low-cost resource that can also be used in a highly efficient manner.
Natural Gas - Natural gas powered boilers are growing in popularity in the United States as advancing drilling technology has created a glut of natural gas. Natural gas also has a relatively high energy-to-heat ratio. This makes a very efficient fuel source.
Oil - Oil-fired boilers have become a more economically viable choice thanks to the U.S. oil boom and the resulting lower cost of oil. Oil-fired boilers are used in some industrial processes as well as for some residential and commercial purposes.
Wood and Bark - Wood and bark burning boilers represent a very popular renewable fuel source solution. Wood and bark burning boilers are especially useful in industries such as paper and pulp which produce wood byproducts as a natural part of their core industrial process.
Alternative Fuels - The use of alternative fuels is gaining momentum in many areas of the energy market and that includes alternative fuel burning boilers. Biodiesel and other bio fuels are rising in prominence as more companies and individuals become environmentally focused.
The Coal System
Coal-fired boilers are one of the most predominant types of boilers used in the power generation industry. Typically, the coal is stored in coal silos until it is needed for use. It is then loaded into a coal feeder which in turns feeds the coal into a coal pulveriser.
As the name implies, coal pulverisers are pieces of equipment designed to pulverise coal before it is used by the boilers. Coal naturally contains some moisture, thus the coal pulverisers grind the coal into a fine powder and then dry out the moisture using built in fans. This makes the subsequent burning of the coal by the boilers much easier and more efficient.
Air and Draft Systems
An extremely important part of the combustion process is establishing and maintaining the correct air-to-fuel ratio. Too much air will require extra, needless heating, while too little air will result in an incomplete burning of the fuel, which both wastes fuel and creates excess smoke and soot.
Because establishing and maintaining the correct air flow is so crucial, most boilers use a mechanical draft system rather than relying on a natural draft, which would be subject to fluctuations. This mechanical draft is created in one of three ways:
Induced Draft - An induced draft may itself be created in one of three ways. The "stack effect" is created when the gas inside the flue is less dense than the surrounding ambient air. This causes the denser ambient air to naturally flow into and through boiler, allowing for combustion.
A second form of induced draft is created with the use of a steam jet or ejector that is directed toward the flue gas. This induces the flue gases to enter the stack and increases the overall gas velocity and draught in the furnace.
The third form of induced draft utilizes an induced draft fan (ID fan). The fan sucks the flue gases out of the furnace and into the stack. These three types of induced drafts generally have a negative pressure.
Forced Draft - A forced draft is created by using a forced draft fan (FD fan) in combination with duct work. The air will typically pass through a heater which warms the air and helps increase the overall efficiency of the boiler. Dampers are generally used to regulate the quantity of air that is allowed into the furnace at a given time. In contrast to induced drafts, forced drafts will generally have a positive pressure.
Balanced Draft - A balanced draft may be created by using the forced draft and induced draft in combination with each other. This cancels out the positive and negative pressure effects of the previous two drafts and maintains furnace pressure at or slightly below atmospheric pressure.
A natural byproduct of coal combustion is ash. This is the leftover inert matter that cannot be burned. Ash must be removed from the boiler and it also shouldn’t be allowed to enter the general atmosphere. Boiler ash can be categorized in two ways:
Bottom Ash - Bottom ash is the portion of ash that remains in the bottom of the furnace. Bottom ash represents roughly about 15% of the total ash. Bottom ash can be collected and transported to storage or disposal sites.
Fly Ash - The remaining portion of ash is dubbed “fly ash.” As the name implies fly ash is present in the air itself, specifically in the flue gases. Fly gas is typically trapped using electrostatic precipitators. Electrostatic precipitators create an electrostatic charge by using high voltage electrodes that are placed in the path of the flue gases. This ionises the ash particles and they are collected on collecting electrodes where they then fall into ash hoppers and can then be removed.
Combustion is one critical process of how boilers are used in power generation. Combustion facilitates the boiler's main function: to boil water into steam. In the next two articles in our power generation series, we will explore the role of turbines and water and steam.