Unit 12: Elements of Boiler Systems Flashcards
Complete Combustion Products?
Carbon Dioxide, Water, & Sulfur Dioxide
What is complete combustion?
Is when enough air, turbulence, temperature, and time is supplied to provide every element enough oxygen to burn completely. Complete combustion allows for the most heat energy to be generated from the fuel being burned.
Non-Combustible elements turn into what?
They turn into ash or pass through the furnace unchanged.
Products of incomplete combustion?
If the conditions for complete combustion are not present then these elements will be produced in the furnace:
1) Carbon Monoxide
2) Free Hydrogen atoms
3) Free Sulfur atoms
What element to look for when testing for incomplete combustion?
Carbon monoxide as it will be the most prevalent of the elements created by incomplete combustion.
What is soot made of?
A product of incomplete combustion of carbon elements that did not bind to oxygen. Both carbon monoxide and carbon dioxide are invisible.
Most common element in atmosphere?
Nitrogen at 78% and then oxygen at 21% for second place
Theoretical Air
The exact amount of air required for complete combustion to take place. Although the theoretical air is never the amount actually used for complete combustion. More oxygen is always required in practice to full expose the fuel elements to the oxygen. The real amount required is called the combustion air and includes the excess air in its calc.
Excess Air
The extra supply of air required for complete combustion to practically occur.
Combustion Air
The amount of actual air required for complete combustion to occur. It is equal to theoretical air + excess air
Percentage of Excess Air Calculation?
Excess air = (Combustion Air - Theoretical Air )/ Theoretical Air
Viscosity
Viscosity is the resistance of a fluid (liquid or gas) to a change in shape or movement of neighbouring portions relative to one another. Viscosity denotes opposition to flow
Refractory
Materials that can withstand very high temperatures
Fuel Oils
Liquid fuels used for boiler firing that derive from petroleum. The petroleum or crude oil is refined through distillation and separated into different hydrocarbons/fuel oils.
Fuel Oil Grade Differences?
There are 6 grades/classes of fuel oils and the first 1 class is the one with the least viscosity, lowest heating value, and lowest flash point. While the 6th class of fuel oils is the most viscous, provides the largest heating value, and has the highest flash point of all the classes.
Ask Clinkers
When fuels are burned they can create soot/ash and this can attach to refractory. Over time it can damage the refractory and peel it off.
Biomass
Are solid biofuels such as:
Hog Fuels Vegetable waste Wood Waste Pulp/Carboard Wood Pellets Municipal Wastes
Solid Fuel Firing Systems
Mechanical Stockers: Feeds solid fuels directly into furnace
Pulverized Fuel Feeder: solids fuels are turned into a fine dust and fed into the furnace through air pipes.
FBC
FBC
Fluidized Bed Combustion:
In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. FBC plants are capable of burning a variety of low-grade solid fuels, including most types of coal, coal waste and woody biomass, at high efficiency and without the necessity for expensive fuel preparation (e.g., pulverising). In addition, for any given thermal duty, FBCs are smaller than the equivalent conventional furnace, so may offer significant advantages over the latter in terms of cost and flexibility.
FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes). The heated precipitate coming in direct contact with the tubes (heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F) have other added benefits as well.
Bubbling FBC
Provides just enough air velocity for the solid fuels and bed materials (Limestone/gravel/sand) to act like a liquid with air bubbles or pockets intermixed. Only small particulates escape from the furnace due to the rate of air velocity. Watertubes are just below the bed materials or in the bed.
Circulating FBC
Has a high air velocity rate (around 6-10 m/s) that circulates solids fuels and the bed materials from the furnace and are recaptured and feedback into the furnace. Smaller particulates are still captured by a cyclone separator and removed allowing for complete combustion of larger pieces of fuel. Generally has a higher combustion efficiency then a bubbling FBC because of the increased fuel to oxygen mixing.
Benefits of FBC
Fuel Flexibility
Smaller Plant size
Less fuel preparation than most other solid fuel furnaces
High combustion efficiency
Improved regulatory compliance as the temperature is too low for NO2 to form and limestone in the bed acts as a desulfurizing agent.
Disadvantages of FBC
Fan power require 3 times the power requirements to reach air velocity
The bed materials need to be replaced
Is not very cost-effective at low operating temperatures or low use.
Mercaptan
Is a compound added to natural gas for leak detection purposes as it can be smelled. It smells like rotten eggs.
