TOIMITTAJAN HUOMAUTUS: Steven A. Jaasund, P.E. is Geoenergy Senior Product Manager for LDX Solutions. He was a co-owner of Geoenergy® and regularly provides his expertise to leaders in the pulp and forest products industry. Jaasund can be reached at sjaasund@ldxsolutions.com
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This article will provide a framework for 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 third part of a three-part series. Part one of this three part series covered combustion basics – the formation of liquids, solids, and gases during combustion. Part two focused on particle formation and behavior. Part three will conclude with a focus on gaseous emission characteristics.
Gaseous Emission Characteristics
There are a number of common gases that occur in biomass combustion. Below are some of the common gases and their characteristics. Some (but not all) of these common gases will be discussed in more detail in this article.
Common gases include the following:
- Carbon dioxide (CO2) is a colourless and acidic gas. It is slightly soluble and denser than
air. - Sulfur dioxide (CO2) is a colourless, acidic gas. It is denser than air.
- Hydrogen chloride (HCl) is a colourless, acidic gas. It is soluble and denser than air.
- Hydrogen (H2) is a colourless, neutral gas. It is highly flammable, insoluble, and less
dense than air. - Oxygen (O2) is a colourless, neutral gas. It supports combustion, is slightly soluble, and
is denser than air. - Ammonia (NH3) is a colourless, alkaline gas. It is very soluble and is less dense than air.
- Carbon monoxide (CO) is a colourless, neutral gas. It is poisonous, soluble, and has a
relatively similar density as air. - Chlorine (Cl2) is a greenish yellow gas. It is soluble and is denser than air.
- Methane (CH4) is a colourless, neutral gas. It is insoluble and is less dense than air.
Hydrogen Chloride
Hydrogen chloride is a colorless gas with a pungent aroma. It has a boiling point of -121°F. The U.S. Environmental Protection Agency (EPA) considers hydrogen chloride to be a hazardous air pollutant.
The Boiler MACT (Maximum Achievable Control Technology) regulates emissions of hazardous air pollutants (HAP) from new and existing industrial, commercial and institutional boilers and process heaters fired with coal, oil, biomass, natural gas or other solid, liquid, and gaseous non-waste materials located at major sources. It regulates hydrogen chloride with a limit of approximately 12 ppm (parts per million).
Hydrogen chloride is very water soluble (720 g/liter at room temperature) and is easily absorbed in alkaline, neutral and slightly acidic solutions. When it is dissolved in water it forms hydrochloric acid which is a strong acid. It can be a source of corrosion, particularly stress corrosion which can lead to cracking and pitting.
Typically, there will be small amounts of hydrogen chloride in flue gases from biomass combustion. This makes it a gas of concern to operators or designers of the biomass combustion facility’s boiler. Especially where Boiler MACT applies.
As mentioned above, hydrogen chloride is very water soluble. This can be a desirable property when controlling emissions because the application of water can ensure that the 12 ppm standard is met.
This standard can be easy to achieve in wet systems. However, wet environments can also have downsides. Notably, hydrogen chloride dissolves to form chloride ions and chloride ions at low pH can be very undesirable on materials, particularly stainless steel. Stainless steel is very susceptible to stress corrosion cracking from chloride, especially at low pH. This means that with a wet absorber either the pH or the chloride concentration in the water needs to be controlled so that stress corrosion cracking doesn’t occur.
Rikkidioksidi
Sulfur dioxide is a colorless gas with a pungent aroma and a boiling point of -98°F. It is not considered a hazardous air pollutant by the EPA. However, it is a criteria pollutant from the Clean Air Act (CAA) of 1970. The CAA identified six common air pollutants of concern. These “criteria pollutants” included the following: ozone, carbon monoxide, nitrogen oxides, sulfur oxides, particulate matter, and lead.
Unlike hydrogen chloride, sulfur dioxide is not very water soluble (94 g/liter). If there is a significant amount of sulfur
dioxide in the gas stream then some alkalinity will need to be added to absorb an acidic species like sulfur dioxide.
Sulfur dioxide does not cause stress corrosion cracking in liquid environments. This is another difference when compared to hydrogen chloride. However, sulfur dioxide is very corrosive if the pH is allowed to get too low.
Whenever there is sulfur and combustion, a small portion will be burned to become sulfur trioxide which is discussed next.
Sulfur Trioxide (SO3)
Sulfur trioxide is a colorless gas with a pungent aroma and a boiling point of 113°F. Sulfur trioxide is very corrosive but is not normally related to stress corrosion cracking.
Sulfur trioxide is a significant byproduct of sulfur combustion. In coal combustion approximately 2% of sulfur becomes sulfur trioxide and in petroleum coke combustion approximately 6% of sulfur becomes sulfur trioxide due to vanadium content.
It may surprise some to learn that the EPA does not consider sulfur trioxide a hazardous air pollutant. It is extremely hazardous but is still not technically considered a hazardous air pollutant. Sulfur trioxide is still unregulated for fuel combustion applications by the EPA.
Sulfur trioxide is highly water soluble. It hydrates readily from the gas phase to form sub-micron particles of sulfuric acid (H2NIIN4). This can be a big problem because sulfuric acid mist is a health-related pollutant.
The formation of sulfuric acid mist is also an issue because—once sulfur trioxide hydrates to form sulfuric acid mist—it is now a particle. Unfortunately, the particle is formed in a size range that is very difficult to collect. Unlike hydrogen chloride, it is not as simple as spraying water on it and absorbing it. When it is a fine particle it is very difficult to control and at the same time it is something that must be controlled.
Sulfuric acid mist from a coal fired power plant
Carbon Dioxide
Carbon dioxide is a colorless, odorless gas. It has a freezing point of -100°F. There are nominal concentrations of carbon dioxide in ambient air, it is now up to 438 ppm.
Carbon dioxide is slightly soluble in water and very soluble in high pH solutions. In water it forms carbonic acid which is mildly corrosive. Because of this, rainwater is approximately pH 5.0 to 5.8.
In a 2007 case the United States Supreme Court were air pollutants under the Clean Air Act and could be regulated by the EPA. This ruling established carbon dioxide as a pollutant. In 2024, the EPA published final carbon pollution standards for power plants that set carbon dioxide limits for new gas-fired combustion turbines and carbon dioxide emission guidelines for existing coal, oil and gas-fired steam generating units.
This shift in regulations indicates that there may be more issues involving carbon dioxide on the horizon. Controlling carbon dioxide may bring large economic consequences. It is definitely something to watch and be aware of in the coming years.
Typen oksidit (NOx)
Nitrogen oxides result from high temperature combustion and/or nitrogen in fuel. A significant form of air pollution occurs when nitrogen oxides react with ozone and sunlight to form photochemical smog (peroxyacetyl nitrate).
Nitrogen oxides have a number of principal forms including the following:
- Nitric oxide (NO), a colorless, odorless gas that is insoluble in water.
- Nitrogen dioxide (NO2), a brown, translucent gas with a pungent odor. It dissolves in water to form nitric acid (HNO3). Unlike all the other gases discussed, NO2 is the only gas that is not colorless.
- Dinitrogen pentoxide (N2O5) is a toxic gas.
- Nitrous oxide (N2O), also known as laughing gas, is the most commonly used sedative today.
Mercury (Hg)
Mercury is both a liquid at room temperature and a metal. It is the only metal that is liquid at room temperature. At a temperature of about 674.1°F – the “boiling point” – it turns to a gaseous form and can be released into the atmosphere.
Mercury is a hazardous air pollutant and is regulated under the Boiler MACT. For biomass stoker type boilers the limit is 5.7 x 10-6 lb/million BTU. This is a very, very low number.
Mercury is also regulated under the EPA’s Mercury and Air Toxics Standards (MATS). These standards adopt emission limits on mercury, acid gases, and other toxic pollutants for affected coal and oil-fired electric utility steam generating units (EGUs).
Controlling mercury emissions will depend on understanding how and where it presents. Mercury emissions occur primarily in the following three forms:
- Elemental, which is pure mercury vapor and is not soluble in water.
- Oxidized is typically HgCl2. It is water soluble and is dominant in high sulfur/high halogen coal combustion.
- Adsorbed is elemental mercury found on the surface of particles with some free carbon.
It is very rare to find significant amounts of mercury in biomass combustion, however it is possible depending on the amount of mercury in the fuel. The best strategy for controlling mercury emissions is to locate the source of the mercury in the fuel and eliminate it. It is also important to carefully monitor the fuel pile so that mercury does not inadvertently end up in the fuel pile.
Key Takeaways
This article has covered combustion basics including common biomass gases. Some of the common biomass gases discussed included hydrogen chloride, sulfur dioxide, and sulfur trioxide. With regard to collection, hydrogen chloride is easy to collect in a wet system while sulfur dioxide needs reagent. Additionally, sulfur trioxide hydrates readily from the gas phase to form sub-micron particles of sulfuric acid. It is also important to note that controlling mercury requires an understanding of its form.
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