Johdanto
This article will provide a framework of understanding the processes that set the stage for
emission control design. Areas of focus include the following:
- Combustion basics
- Particle formation and behavior
- Gaseous emission characteristics
This is the first part of a three-part series. Please note, this three-part series will not focus on carbon monoxide. Here, in part one, the discussion will focus on combustion basics.
Combustion Basics
There are three states of matter: solids, liquids, and gases. Everything around us is either a
solid, a liquid, or a gas.
A corollary to all this is the word aerosol. The term aerosol is often used improperly. For many, aerosol is associated with what comes out of aerosol cans. An aerosol, properly defined, is a suspension of particles in a gas. It doesn’t necessarily have to be a liquid, it can be a liquid or a solid
There are four principal gases that come from the combustion of biomass. The following four equations represent at a very basic level what goes on in biomass combustion and the resulting gases. The following three reactions are exothermic (they give off heat):
C + O2 → CO2 + heat
- Carbon plus oxygen makes carbon dioxide and releases heat
C + 1⁄2O2 → CO + heat
- Carbon plus deficiency of oxygen gives carbon monoxide plus heat
H2 + 1⁄2O2 → H2O + heat
- Hydrogen and oxygen burn to create water vapor and heat
The fourth reaction is an endothermic reaction (it absorbs heat). It is N2 + 1⁄2O2 → NO – heat. This reaction can occur when there is enough temperature, over 1500°F or 1600°F. When the temperature is hot enough nitrogen will react with oxygen to form nitric oxide. There are also secondary processes that also result from the combustion of biomass. Those processes can generally be categorized as the following:
- Thermal processes
- Chemical processes
- Mechanical processes
These secondary processes as well as ash will be discussed below.
Tuhka
Ash is also produced during biomass combustion. Ash is what is leftover after combustion, it is the stuff that does not burn up.
There are two types of ash: bottom ash and fly ash. It is the same ash just in two different forms. Bottom ash, as its name implies, is the ash that falls to the bottom. Fly ash is the ash that goes up with the combustion products. It is the overall term used to describe the particles that go out of a stack.
Wood Ash Characteristics
There are different types of biomass but much of biomass is wood. The burning of hardwoods like oak and maple typically results in an ash content that is about 2 to 5% of the total weight. This is a somewhat higher ash content when compared to other woods like the southern yellow pine, which typically results in an ash content that is about 1 to 3% of the total weight. Southern yellow pine is considered a softwood lumber and is the raw material for lumber and plywood.
Chemistry
Ash can be broken down into its general characteristics. Its chemical composition is highly variable. The elements found in biomass can be termed species. Volatile species can be species that melt at relatively low temperature. If raised to a high enough temperature, the ash will tend to melt and fuse like melted glass. The volatile species that melt at a relatively low temperature and then evaporate at somewhat lower temperatures are the principal components in wood ash, which include sodium and potassium.
There are also less volatile species which are found as part of wood ash. Less volatile species include calcium and magnesium as well as small amounts of iron, aluminum, and zinc. All of these can be found in small amounts in wood ash.
Silica (otherwise known as sand) can also be found in wood ash. This is interesting to note because wood does not actually have much if any silica in it at all. It is the processing of wood that brings in the silica. For example, when wood is burned it gets transported or dragged along the ground and gets dirt on it. Consequently, the fly ash that comes out of a combustion source like a boiler will have a lot of silica in it.
At least one hazardous species can be found in more trace amounts in ash. That hazardous species is manganese (Mn). The U.S. Environmental Protection Agency has determined that manganese is a hazardous air pollutant. It has particularly toxic effects on human health.
Other Ash Characteristics
Beyond volatility, there are other characteristics of ash from biomass. Ash from biomass is generally very abrasive. This can cause issues when transporting it at higher velocities, whether it is mixed with water and sluiced away from a boiler operation or pneumatically conveyed in an airstream. The ash in either instance can be very abrasive and wear out parts and pieces including nozzles, pipes, and pumps.
Additionally, ash from biomass is universally alkaline. If the ash is dissolved in water, that alkalinity will present as high pH.
Thermal Processes
One way that emissions form are thermal processes. The thermal process is the combustion going on in the boiler. Different particles like fly ash can come from the thermal process. Some of the general ways that emissions can come from the thermal processes include the following:
- Potassium oxide decomposes at 350°C
- Sodium oxide has a melting point of 1132°C and a boiling point of 1275°C
- Potassium chloride has a melting point of 770°C
- Sodium chloride has a melting point of 801°C
Some species in a flame are vapors, like sodium and potassium. Sodium and potassium basically melt in the flame of a fire. After they melt, they vaporize within the flame. Then as the flame goes out and the gases proceed away from the flame and the gases cool, those vapors cool to form droplets and later solid particles. This is how sodium and potassium melt and recombine as oxides in the combustion process.
A similar compound is potassium chloride. Chlorine can get into wood waste that is being burned in a biomass boiler from the logging transportation process. For example, along the seacoast logs are often floated in the water and then dragged to the mill via tugboat.
Those logs that are floated can have a lot of sodium chloride in them. Sodium potassium and sodium chloride can be formed from that thermal process where that ash is melted and vaporized and recombines to become a chloride.
Chemical vapor phase reactions can also happen during the combustion process and create other elements. If there is chlorine in the biomass then hydrogen chloride (HCl) tends to form. It is the following reaction: H+ + Cl- → HCl. If there is any mercury (Hg) and chlorine in the gastream then mercuric chloride (HgCl2) can form. It is the following reaction: Hg+2 + Cl- → HgCl2. All of these elements are examples of thermally created elements that come from a thermal process.
Mechanical Processes
There are also mechanical processes that can result in the creation of fly ash. Some of these processes include the following: grinding, abrasion, shearing, pulverizing, and aerodynamic entrainment. These are all processes that can create particles.
Additional Secondary Processes
There are also additional secondary processes or reactions that can occur. These processes are not thermal and are not mechanical actions. These processes can include the following: downstream reactions, chemical reactions, hydration reactions, and condensation downstream of the control device.
The chemical reactions can include the following:
- SOx (gas) + NH3 (gas) → (NH4)2SOx (particle)
- HCl (gas) + NH3 (gas) → NH4Cl (particle)
Hydration reactions can include the following:
SO3 (gas) + H2O (gas) → H2SO4 (particle)
- Sulfur trioxide gas in the presence of water vapor will suck the water out of the air and form sulfuric acid mists.
Condensation downstream of control device can include the following:
Organic vapor -Δ (heat) → organic particle
- When heat is removed from organic vapor the result is an organic particle.
Next Up: Particle Formation and Behavior
This article has covered combustion basics including how things can form whether they are liquids, gasses, or solids. This has included an overview of the origin of the particles as well as an overview of the processes that create the particles including thermal, mechanical, and additional secondary processes. Next up will be part two of this three part series which will include a deeper look into particle formation and behavior.
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