Introduction

Power Generation, Transmission, and Use

Markets, Regulation, and Oversight

Impacts of Power Generation and Transmission

Looking Ahead

Appendices

CEIR Report Map

PPRP Home

Maryland Power Plants and the Environment (CEIR-18)

4.6 Power Plant Combustion By-Products

The combustion of coal to produce electricity yields solid coal combustion by-products (CCBs), which in the past were often disposed of in landfills. Fortunately, CCBs can be used in innovative ways to reduce disposal and serve a wide variety of purposes. This section of the report focuses on the generation of CCBs at coal-fired power plants in Maryland and describes ongoing research efforts to promote beneficial use applications for CCBs. The ultimate goal is that all CCBs generated in Maryland will be used in environmentally beneficial or benign ways.

4.6.1 CCB Generation and Characteristics

In 2014, coal-fired power plants in Maryland generated an estimated 1.5 million tons of CCBs, as reported to the Maryland Department of the Environment (MDE). The term CCBs includes several solid materials with different physical and chemical characteristics. The types and percentages of CCBs generated in Maryland are shown in Figure 4-36.

Figure 4-36 CCBs Produced in Maryland in 2014

Figure 4-36 pie chart showing CCBs in MAryland

The exact chemical nature of CCBs depends upon the nature of the coal burned, the combustion process used, and any emission control processes used. Most power plants in Maryland burn bituminous coal from the eastern United States and produce predominantly Class F fly ash and bottom ash. Fly ash and bottom ash are distinguished by their physical characteristics. Fly ash is composed of very fine, and generally spherical, glassy particles that are fine enough to be transported from the furnace along with emission gases and are captured in electrostatic precipitators or baghouses. Bottom ash is composed of coarser, angular, and porous glassy particles that are heavier and thus fall to the bottom of the furnace, where they are collected. Boiler slag is a specialized type of bottom ash that collects in a molten form and is entirely glassy. There is little difference in the chemical makeup of fly ash and bottom ash. Class F ash is primarily composed of silicon, aluminum, and iron oxides, making it an excellent pozzolan material (meaning that it contributes to cementitious reactions when combined with water and free lime). It may also contain trace metals such as titanium, nickel, manganese, cobalt, arsenic, and mercury. For this reason, electric utilities are required to include all applicable constituents of their CCBs when reporting chemical releases to EPA’s Toxics Release Inventory (TRI) program, which maintains a database listing the quantities of toxic chemicals released to the environment annually by various industries. When fly ash is used as pozzolan to produce solid material, its potential to leach trace elements is greatly reduced.

The composition of Class F fly ash and bottom ash is further altered by emission control technologies, like low NOx burners. These burners reduce the emission of smog-producing nitrogen oxides from power plant emissions, but they also tend to result in CCBs with higher levels of unburned carbon (also known as loss-on-ignition or LOI). High LOI material cannot be used by most cement manufacturers and ready-mix concrete industries. Maryland power plants have overcome this problem by adopting CCB beneficiation technologies. There are two fly ash beneficiation plants in Maryland, the STAR plant and the STET plant (formerly known as the STI plant) (Figure 4-37). These two plants use different technologies to reduce the level of unburned carbon in fly ash, making it highly desirable for the cement and concrete industries.

Figure 4-37  STET and STAR Fly Ash Beneficiation Plants

STET (Formerly STI) Facility

STAR Facility

Alkaline CCBs are fly ash and bottom ash materials with high levels of calcium and high pH values. Class C fly ash and fluidized bed combustion (FBC) ash are two alkaline ashes produced in Maryland. The C.P. Crane plant uses sub-bituminous coal from the Powder River basin. This coal contains more calcium carbonate than eastern coals and results in Class C ash. The AES Warrior Run power plant near Cumberland uses fluidized bed combustion (FBC) technology in which coal and finely ground limestone are fed into the combustion chamber and mixed by forcing in air. The heat in the combustion chamber causes the limestone to decompose to an oxide that captures SO2. FBC units can remove more than 95 percent of the sulfur produced from burning coal and the resulting FBC material by-products are similar to Class C ash. Alkaline CCBs often have self-cementing properties because they contain calcium oxide (free lime). However, they can also contain high levels of magnesium, which can interfere with some beneficial use applications.

The third category of CCBs produced in Maryland is flue gas desulfurization (FGD) materials. Like FBC processes, FGD uses limestone as a sorbent to control sulfur emissions. Unlike FBC processes, the sorbent is introduced, not with the coal, but into the exhaust system, producing a completely separate stream of residuals with a distinctive composition. FGD materials consist almost entirely of calcium sulfate, and are often referred to as synthetic gypsum. FGD scrubbers were installed at the Brandon Shores, Dickerson, Chalk Point, and Morgantown power plants in 2010.

If not managed in accordance with sound engineering principles, landfilled CCBs have the potential to adversely impact Maryland’s terrestrial and aquatic resources. Careful planning and execution of the disposal and/or use of CCBs is necessary to minimize impact to the surrounding environment. The importance of sound engineering and proper placement of CCBs was highlighted at the BBSS Mine Reclamation Site. Between 1995 and 2007, Constellation Power disposed of 200,000 to 400,000 tons of CCBs, primarily unstabilized Class F fly ash, at a sand and gravel mine reclamation site in Anne Arundel County owned by BBSS, Inc. The site relied on a natural soil cover and its underlying geology to minimize the potential for leachate to impact the regional ground water system.

In 2006, MDE requested that PPRP provide assistance on an independent evaluation of the source of heavy metals and dissolved sulfate detected in residential wells near the site. A statistical comparison of residential and monitoring well water quality data indicated that fly ash placement in the Turner and Waugh Chapel Pits likely contributed to the deterioration of ground water quality nearby. The site continues to be an issue from the standpoint of contaminating local wells; EPA included the BBSS site in a list of documented damage cases related to CCBs, when it published final regulations on CCB disposal in 2015. Constellation and MDE entered into a Consent Decree in October 2007 with an approach to resolve the identified impacts.

 

As classified by the American Society for Testing and Materials (ASTM), Class F fly ash is distinguished from Class C fly ash by having less than 10 percent calcium (expressed as CaO) by weight.
Pozzolan is a type of material containing silicon and aluminum oxides that reacts with portland cement and water to form concrete.
As classified by the American Society for Testing and Materials (ASTM), Class F fly ash is distinguished from Class C fly ash by having less than 10 percent calcium (expressed as CaO) by weight.