Pollution Control in Boilers

In recent years (data available to 2017), the United States saw significant improvements in air quality across the entire country. All of the major pollutants decreased in concentration over the study period including sulfur dioxide (SO2) down 79%, nitrogen dioxide (NOx) down 35%, and PM2.5 particles down 41%. The United States achieved these reductions through new environmental legislation, enforcement of existing laws like the Clean Air Act, and the advancement of pollution control technologies all while the nation’s economy grew and industrial capacity expanded.

As identified in an Environmental Protection Agency (EPA) report, a major source of air pollution is the burning of fossil fuels in industrial factories or boilers for power generation. The EPA warns that residuals from burning coal (coal ash) are “one of the largest types of industrial waste generated in the United States.” Burning coal is an inherently dirty process and federal limits restrict the concentrations of SO2, NOx, and PM2.5 discharged to the atmosphere. Modern boiler design and pollution reduction technologies are available so that plants can provide heat or electrical power at reasonable costs while still being compliant with environmental regulations.

Pollution control in coal-fired boilers can occur at all stages of the combustion process: pre-combustion, during combustion, and post-combustion.

Pre-Combustion

The selection of fuel to burn in the boiler is the first opportunity to reduce flue gas contaminants. For example, natural gas burns cleaner than fuel oil, medical waste, or biomass and produces the least amount of pollutants in its flue gas. Natural gas is considered a low-nitrogen fuel and yields very little NOx as a by-product of combustion. Also, as the EPA notes in its technical bulletin on controlling NOx that “natural gas is desulfurized before it is sent in a pipeline. Therefore, natural gas has almost no sulfur, essentially no impurities, and no ash.”

During combustion

The moment fuel and air ignite presents an opportunity to fine-tune the stoichiometry of the combustion process and the by-products of that reaction. With flue gas recirculation (FGR) a portion of the flue gas is sent back to the combustion chamber diluting the oxygen level of the combustion air, reducing the core flame temperature. The reduced temperature lowers the NOx level in the flue gases to below 20%. Cleaver-Brooks describes FGR as “the most effective and popular low NOx technology for firetube and watertube boilers. “

A fluidized bed recirculation (FBR) boiler burns crushed coal, wood or other low-grade fuels sitting on a sand bed within the boiler fireside. Combustion air is blown up through the bottom of the sand bed and mixes with the fuel on top of the sand where combustion occurs. This boiler design results in a fast mix of air and fuel and encourages rapid heat transfer. The burning environment gives a cleaner burn with reduced quantities of NOx and SO2 in the flue gases. SO2 emissions can be reduced further by introducing pulverized limestone into the combustion chamber. Sulfur in the flue gasses reacts with the limestone to form gypsum that, when separated from the fly ash, can be used to make other saleable products.

Post-Combustion

Coal ash leaving the combustion chamber can be captured and particulate pollutants removed before being released to the atmosphere.

One method for capturing fine particles like PM2.5 is to pass boiler flue gases through an electrostatic precipitator (ESP). With an ESP the flue gas flows across high-voltage wires that electrically charge fine particles suspended in the gas. The charged particles are attracted to a collection electrode, such as a series of metal pipes or plates, and accumulate on the electrode surface. ESP’s remove more than 95% of PM2.5 particles from coal flue gas and more complex ESP systems have removal efficiencies approaching 99%. To maintain such high levels, the collection electrodes should be cleaned per the manufacturer’s recommendations to minimize the thickness of fly ash buildup on the surface.

Cyclonic or cyclone fly ash separators (not to be confused with a cyclone furnace) are another method to remove fine particles from coal boiler flue gas. A cyclone separator works by blowing flue gasses against the interior wall of a cylindrical vessel. The gas enters the vessel near the top and circulates inside like a tornado. The centrifugal forces of the rotating flue gas push the fine particles to the outside of the “tornado.” The particles hit the interior surface and fall to the bottom to be collected while clean flue gas exits the vessel at the top. Although the efficiency of a cyclone separator may reach 90%, they are rarely used alone to meet EPA standards for particulate discharge. Most cyclone separators are used in conjunction with other technology like an FBR boiler to achieve the desired particulate concentrations. The clean flue gas can be analyzed to provide insight into the overall efficiency of the boiler and heat transfer processes inside.

The EPA’s 2018 report Our Clean Air summarizes things nicely, “The U.S. leads the world in having clean air and a strong economy due to implementation of the Clean Air Act and technological advancements from American innovators.” The power industry, pollution control manufacturers, and the EPA have a strong record of working together to create reliable electrical power while reducing atmospheric contaminants and improving the quality of the air we .

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Goodway’s free webinar, “Industrial Boiler Cleaning” is available. The webinar host, Mark Roth, Goodway sales director, with guest Ray Field, Goodway’s director chemical technologies, walk through the basics of industrial boilers, the importance of cleaning and preventative maintenance, how to determine whether mechanical or chemical cleaning is the best solution for your facility, and the process for performing those cleaning options. There is also a question and answer session at the end of the webinar. Don’t miss this important webinar!

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Over the years the complexity of HVAC equipment has increased, but many basic maintenance tasks like cleaning haven’t changed very much. Luckily for technicians, the tools needed to perform this work have improved, and dirty jobs like cleaning, pressure washing, or descaling can be completed faster than ever before.

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