Handbook Of Pollution Control And Waste Minimization - Chapter 20 potx

Dick The Ohio State University, Wooster, Ohio 1 INTRODUCTION AND BACKGROUND During coal-fired electric power production, four main types of coal combustionby-products CCBs are produced:

Minimization and Use of Coal Combustion By-Products (CCBs): Concepts and

Applications

Harold W Walker, Panuwat Taerakul,

Tarunjit Singh Butalia, and

William E Wolfe

The Ohio State University, Columbus, Ohio

Warren A Dick

The Ohio State University, Wooster, Ohio

1 INTRODUCTION AND BACKGROUND

During coal-fired electric power production, four main types of coal combustionby-products (CCBs) are produced: fly ash, bottom ash, boiler slag, and flue gasdesulfurization (FGD) material (1,2) In 1998, 97.7 million metric tons of CCBswere produced in the United States (see Figure 1) Fly ash was generated in thelargest quantity (57.1 million metric tons), with FGD material the second mostabundant CCB (22.7 million metric tons) Roughly 15.1 million metric tons ofbottom ash were generated and 2.7 million metric tons of boiler slag wereproduced Although the majority of CCBs produced currently enters landfills andsurface impoundments, there is great potential for the effective and environmen-tally sound utilization of these materials

Currently, the amount of CCBs entering landfills and surface ments is greater than half of the total municipal solid waste (MSW) disposed of

impound-in the United States (see Table 1) Of the 97.7 million metric tons of CCBsgenerated in 1998, 69.4 million metric tons of CCBs (or 70%) were disposed of

in landfills or surface impoundments (1) In 1997, the most current year for whichdata are available, the total MSW disposed of in landfills was 119.6 milliontons (3) The amount of CCBs disposed each year is greater than the amount ofpaper (37.4 million metric tons), plastic (15.5 million metric tons), wood (8.4million metric tons), and glass (6.9 million metric tons) discarded

57.1

15.1

2.7 22.7

97.7

0 20 40 60 80 100 120

Fly Ash Bottom

Ash

Boiler Slag

FGD Total CCPs

F IGURE 1 CCB production in million metric tons in the United States in 1998 (1).

T ABLE 1 Amount of CCBs Disposed of in

Landfills in the United States in 1998 Compared

to Disposal of Municipal Solid Waste (MSW)a

Material Metric tons × 10 6 Reference

a Data for disposal of MSW are for 1997, the most current

year for which data are available.

Recently, the American Coal Ash Association (ACAA) proposed that CCBs

be considered a product, and therefore they recommend that these materials bereferred to as coal combustion products (CCPs) Considered as a commodity,CCBs are ranked as the third largest nonfuel mineral commodity produced in theUnited States (1,4) As shown in Table 2, the amount of CCBs generated everyyear exceeds the amount of Portland cement generated in the United States, issignificantly greater than the production of iron ore, and falls behind the produc-tion of crushed stone, sand, and gravel

The purpose of this chapter is to review the current state of the art intechnology for minimizing CCB generation, maximizing CCB use, and reducingthe disposal of CCBs in landfills and surface impoundments This chapter willfirst present a review of important federal regulations influencing the generationand utilization of CCBs in the United States Next, the physical, chemical, andengineering properties of CCBs will be discussed, and the operational factorsaffecting CCB generation will be presented The chapter will conclude with adiscussion regarding strategies for minimizing CCB production and maximizingthe utilization of CCBs Potential barriers to utilization and minimization in thefuture will also be discussed

2 FEDERAL REGULATIONS INFLUENCING CCB

GENERATION AND USE

Governmental regulations of emissions from electric power plants combined withefforts to improve air quality have had a profound effect on the amount and type

of CCBs produced in the United States over the past 25 years The Clean Air Act

of 1967 was the first legislation to establish the authority of the federal ment to promulgate air quality criteria (5) It set the groundwork for future

govern-“technology-forcing legislation,” i.e., legislation that sets standards unattainable

T ABLE 2 Amount of CCBs Produced in the United

States in 1998 Compared to Traditional Nonfuel

utilizing existing technology This regulatory approach required industry andutilities to develop new technologies to meet promulgated standards.

The Clean Air Act Amendments of 1970 established Natural Ambient AirQuality Standards (NAAQS) and set specific pollutant removal requirements(New Source Performance Standards or NSPS) for both stationary and mobilesources (5) NSPS, which are applicable to coal-fired utilities, were written in part

60, subpart D, Da, Db, and Dc of 40 CFR (Code of Federal Regulation) (6) NSPS

in 40 CFR, part 60, subpart D, set air pollutant levels for coal-fired steam

generators with heat input rates over 73 megawatts (MW), constructed or stantially modified after August 17, 1971 Amendments to the Clean Air Act in

sub-1990 added new provisions to reduce the formation of acid rain by decreasingsulfur and nitrogen oxide emissions Key to these provisions was the requirement

to reduce annual SO2 emissions by 10 million tons below 1980 levels, and toreduce NOx emissions by 2 million tons below 1980 levels To achieve theseemission reductions, the Clean Air Act Amendments of 1990 promulgated NOxand SO2 performance standards and set up an innovative emission trading systemfor SO2 reduction In phase I of the SO2-reduction program, the legislationrequired 110 identified utilities to reduce SO2 emissions to 2.5 lb/mmBTU byJanuary 1995 Phase II mandated further reductions in emissions to 1.2lb/mmBTU for all utilities generating at least 25 MW of electricity It is estimatedthat phase II requirements will affect 2128 utilities in the United States (7) The

NOx reduction program was also separated into two phases In phase I, Group 1boilers (dry-bottom wall and tangentially fired boilers) were required to meet NOxperformance standards by January 1996 (8) Phase II set lower NOx emissionlimits for Group 1 boilers and established initial NOx emission limitations forGroup 2 boilers (cell burner technology, cyclone boilers, wet bottom boilers, andother types of coal-fired boilers) (7)

To meet these federal regulations, coal-fired utilities have switched toalternative fossil fuels or installed air pollution control technologies such aselectrostatic precipitators, baghouses, and wet or dry SO2 scrubbing systems.Currently, CCBs generated as a result of air pollution control processes areregulated under subtitle D of the Resource Conservation and Recovery Act(RCRA), which pertains to nonhazardous solid wastes (9) In 1988, and thenagain in 1999, the U.S Environmental Protection Agency (EPA) issued a Report

to Congress examining the environmental impacts associated with CCB use anddisposal (10,11) Reports in both 1988 and 1999 concluded that CCBs werenonhazardous and nontoxic materials In early 2000, based on its own findings inthe Report to Congress as well as input from environmental groups, the EPAmaintained its previous ruling that CCBs will continue to be regulated undersubtitle D of the RCRA As a result, the use and/or disposal of CCBs is regulated

at the state level For example, regulations in the state of Ohio consider fly ash,

bottom ash, boiler slag, and FGD generated from coal or other fuel combustionsources to be exempt from regulation as hazardous waste (12).

3 PHYSICAL, CHEMICAL, AND ENGINEERING

PROPERTIES OF CCBS

Information regarding the physical, chemical, and engineering properties ofCCBs is required before these materials can be safely and effectively utilized Thephysical and engineering properties, in particular, are important parametersaffecting the behavior of CCBs in various engineering applications Informationregarding the chemical composition is important for addressing potential environ-mental impacts associated with CCB utilization and disposal Chemical data arealso useful for explaining physical properties when pozzolanic or cementitiousreactions take place

As mentioned above, the four main types of CCBs are fly ash, bottom ash,boiler slag, and FGD material Fly ash is a powdery material removed fromelectrostatic precipitation (ESP) or baghouse operations, while bottom ash is agranular material removed from the bottom of dry-bottom boilers Boiler slag is

a granular material that settles to the bottom of wet-bottom and cyclone boilers

It forms when the operating temperature in the boiler exceeds the ash fusiontemperature Boiler slag exists in a molten state until it is drained from the boiler.The majority of FGD material is a mixture of fly ash and dewatered scrubbersludge Scrubber sludge is produced when flue gases are exposed to an aqueoussolution of lime or limestone The wet scrubber sludge is dewatered and stabilizedwith fly ash and extra lime Alternatively, the scrubber sludge can be oxidized tocalcium sulfate (CaSO4) to produce synthetic FGD gypsum Dry FGD processesare widely used, in which limestone is injected directly into the boiler or flue gasstream Dry FGD by-products are removed from the flue gas by electrostaticprecipitation or baghouse operations

3.1 Physical and Engineering Properties of CCBs

A number of the physical and engineering properties of fly ash, bottom ash, boilerslag, and FGD material are summarized in Table 3 (10,11,13–16,18,19) Fly ash

is usually spherical, with a diameter ranging from 1 to 100 µm Fly ash has theappearance of a gray cohesive silt and has low permeability when compacted.Bottom ash and boiler slag are granular in shape, with sizes ranging from 0.1 to10.0 mm Boiler slag has a glassy appearance Bottom ash has a permeabilityhigher than fly ash, while boiler slag has a permeability similar to that of coursesand Fly ash, bottom ash, and boiler slag have dry densities that range between

40 and 100 lb/ft3 (10,11,15,16) Fly ash has lower shear strength than both bottomash and boiler slag

The physical characteristics of FGD material depend on the type of FGDsystem used: wet or dry (see Table 3) Wet FGD systems generate by-productswith diameters ranging from 0.001 to 0.05 mm Dry FGD systems produceby-products with diameters ranging from 0.002 to 0.074 mm FGD materialgenerally has low permeability, ranging from 10–4 to 10–7 cm/s The unconfinedcompressive strength is affected by the moisture content of FGD, and thepercentages of fly ash and lime For example, wet FGD scrubber sludge is similar

to toothpaste in consistency and has little unconfined compressive strength.However, the strength of wet FGD is greatly improved when FGD sludge isstabilizing by mixing with lime and fly ash

3.2 Chemical Properties of CCBs

The chemical characteristics of fly ash, bottom ash, and boiler slag depend greatly

on the type of coal used and the operating conditions of the boiler (10,11) Over95% of fly ash consists of oxides of silicon, aluminum, iron, and calcium, withthe remaining 5% consisting of various trace elements (10,11) The chemicalcomposition of fly ash is affected by the operating temperature of the boiler,because the operating temperature influences the volatility of certain elements.For example, sulfur may be completely volatilized at high temperature andremoved during lime scrubbing, thus reducing the amount in the fly ash, bottomash, and boiler slag (10,11)

Table 4 shows the trace-element content of fly ash, bottom ash, boiler slag,and FGD material (10,11,20) The elemental composition of fly ash from two

T ABLE 3 Summary of Physical Characteristics and Engineering Properties

of Fly Ash, Bottom Ash, Boiler Slag, and FGD Material (10,11,13–16,18,19)

Physical characteristics Fly ash

Dry density (lb/ft3) 40–90 40–100 56–106 64–87 Permeability (cm/s) 10–6–10–4 10–3–10–1 10–6–10–4 10–7–10–6Shear strength

T ABLE 4 Trace Element Composition of Fly Ash, Bottom Ash, Boiler Slag (10), and FGD Material (20)

Element (mg/kg)

types of collection methods is shown Mechanical collection methods generallycollect larger particles from the flue gas, while finer ash particles are collected byelectrostatic precipitators (ESPs) or baghouses However, similar ranges of mosttrace elements are found in both types of collection methods Some exceptions tothis are arsenic, boron, lead, and selenium, which may be found at slightly higherfractions in fly ash collected by ESPs or baghouses Cadmium and fluorine may

be present at higher levels in ash collected by mechanical methods

The chemical characteristics of FGD by-products depend on the type ofabsorbent used and the sulfur content of the coal In the United States, approxi-mately 90% of FGD systems use lime or limestone as a sorbent (17) Inlime-based FGD processes, the absorbent reacts with sulfur in the flue gas andforms a calcium compound, either calcium sulfite or calcium sulfate, or a calciumsulfite–sulfate mixture (10,11) In systems that use dual-alkali scrubber technol-ogy, sodium hydroxide, sodium sulfite, or lime is used as absorbent solution.These types of systems generate calcium sulfite and sodium salts (10,11) Inspray-drying scrubber systems, sodium sulfate and sodium sulfite are producedwith sodium-based reagents When fly ash is added to FGD, the quantity andcharacteristics of the fly ash will also affect FGD chemical characteristics.The most significant components in FGD include calcium and sulfur, withlesser amounts of silica, aluminum, iron, and magnesium if fly ash is added Theelemental composition of dry FGD materials has been determined based on datafrom a variety of dry-scrubber technologies, including spray dryer systems, ductinjection, lime injection multistage burner (LIMB) processes, and a number offluidized bed combustion (FBC) processes (i.e., bed-ash process and cyclone ashprocess) (18,19) The calcium content of dry FGD material varies in the rangefrom 10% to 30% depending on the particular scrubber technology The sulfurcontent of dry FGD material typically varies between 4% and 11% The siliconcontent of dry FGD may range from 2% to 11%, while the aluminum content canvary from 1% to 7% Table 4 shows the trace-element content of dry FGDmaterials (20) Although detectable amounts of arsenic, cadmium, chromium,copper, lead, molybdenum, nickel, selenium, and zinc are present in dry FGDmaterials, levels of these constituents are typically lower than EPA land applica-tion guidelines for sewage sludge

For many CCB applications, it is important to understand the leachingbehavior of these materials The EPA’s Toxicity Characteristic Leaching Proce-dure (TCLP) is a commonly used method for characterizing the leaching potential

of organics, metals, and other inorganic constituents from CCB matrices (21).Table 5 shows the results of TCLP analyses of dry FGD materials and ashproduced from various air pollution control technologies Typically, very lowlevels of organic materials are found in CCBs, and therefore, TCLP tests focus

on examining the leaching behavior of inorganic constituents The TCLP valuesfor FGD shown in Table 5 were determined for a variety of dry scrubber

T ABLE 5 Range of Values Observed for TCLP Analysis of Dry FGD Materials (19,20) and Ash (14)

technologies TCLP leachate typically meets most primary and secondary ing water standards Levels of silver, arsenic, barium, cadmium, copper, iron,mercury, manganese, nickel, phosphorus, antimony, and zinc in leachate aretypically below the limit of detection For all FGD materials shown in Table 5,high pH values are observed, thus making FGD an attractive product for applica-tions requiring alkaline materials Typically, with the exception of sulfur andcalcium, higher levels of most inorganic elements are found for TCLP testscarried out with ash than for FGD It should be noted that the acidic conditionsand high liquid-to-solids ratio of the TCLP test are perhaps more favorable forleaching than conditions typically observed in field applications.

drink-4 FACTORS AFFECTING CCB GENERATION

The physical and chemical properties of CCBs and the quantity of CCBsproduced will depend on the mechanical design and operation of the combustionprocess, the type of air pollution control equipment utilized, as well as thecharacteristics of the coal used in the combustion process (11) In order tominimize CCB generation, it is important to understand how these factors affectthe type and amount of solid by-product produced In all cases, however, efficientenergy production and low-pollutant air emissions must be maintained

The most widely used boiler technology is the PC boiler The coal used in

PC boilers is finely ground prior to combustion The large effective surface area

of finely ground coal used in PC boilers increases combustion efficiency Thegreater efficiency of combustion reduces the total volume of ash by-productsproduced There are two types of pulverized coal boilers; wet-bottom anddry-bottom boilers The larger-sized ash that falls to the bottom in a dry-bottomprocess remains dry and becomes bottom ash For the wet-bottom process, ash isremoved as a flowing slag Large ash particles fall to the bottom of the furnaceand flow out of the furnace in a molten state which later solidifies as slag (10,11)

As seen in Figure 2, dry-bottom PC boilers produce 80% fly ash and 20% bottomash PC boilers with a wet-bottom design produce 50% fly ash and 50% slag Thepredominance of fly ash in these two types of boilers is primarily a result of thesmall particle size of ground coal used in the combustion process

Stoker boiler technology is typically used in smaller utility plants A stokerboiler is classified based on the location of the stoker, the method of coal feeding,and the method of grating coal in the furnace Spreader stokers are the mostwidely used of all stoker technologies (10,11) The bottom ash generated byspreader stokers ranges from free-flowing ash to fused slag, while the bottomash created from other types of stokers is normally slag (10,11,22) Figure 2shows that spreader stokers produce 35–60% fly ash and 40–65% bottom ash andslag Other types of stokers produce about 10% fly ash and 90% bottom ashand slag.

Cyclone boilers are used for coal combustion and are designed to circulateair to enhance the combustion of fine coal particles in suspension This designhelps to reduce erosion and fouling problems in the boiler Larger ash particlesstick to the molten layer of slag and flow out Combustion occurs in a horizontalcylindrical vessel attached to the boiler This kind of design facilitates the flow

of molten slag and also reduces the cost of particulate collection (10,11) Most ofthe by-product from the cyclone design is in the form of slag Figure 2 shows thatcyclones produce 30% fly ash and 70% slag

CycloneOther Stoker

SpreaderStokerWet PC BoilerDry PC Boiler

% Ash ProportionFly Ash Bottom Ash/Boiler Slag

F IGURE 2 Approximate ash distribution as a function of boiler technology (10,11,22).

Other technologies are also used for coal combustion These alternativeboilers may also aid in controlling air emission Fluidized bed combustion (FBC)

is a boiler technology that can be used with a variety of fuels (22) Thistechnology has a high combustion efficiency at low operating temperatures (11).The fluidized bed combustion system consists of a blower that injects preheatedair into the fluidization vessel, and a bed material that can be sand or a reactivesolid Injection of air into the vessel fluidizes the bed material and aids incombustion The amount of CCBs produced from FBC is based on the type ofFBC Two types of FBC include bubbling fluidized bed systems and circulatingfluidized bed systems

Bubbling fluidized bed systems have gas velocities of 5–12 ft/s Gas flowpasses through the bed and causes the bed material to “bubble.” In bubbling FBCsystems, the particle size of bottom ash in the bed is usually larger and is packeddenser (~45 lb/ft3) than in circulating FBC systems (11,22) CCBs generated frombubbling FBC systems include ash, sand, and other inert bed material Lime orlimestone may be added directly to the bed to aid in sulfur emission control(11,22) As a result, by-products from FBC boilers may also contain unreactedlime, calcium sulfate, and/or calcium sulfite

Circulating fluidized bed combustion systems have higher gas velocities ofabout 30 ft/s In circulating systems, some of the bed material is recovered fromthe gas phase and reinjected into the fluidized bed vessel Bottom ash in the bed

of circulating FBC systems is usually finer and more densely packed (~35 lb/ft3)than in bubbling FBC systems (11,22) Ash generated from circulating FBCsystems consists mainly of fly ash, with lesser amounts of bottom ash (22)

4.2 Air Pollution Control Technology

The type of technology used for controlling pollutants released to the atmosphereduring coal combustion influences the generation and characteristics of CCBs.There are two main categories of air pollution control technologies that generateCCBs during coal combustion: particulate control and gaseous emission controltechnologies

Particulate control technologies during coal combustion capture fly ashfrom the flue gases before they are released to the atmosphere The processesmost often used for particulate control are electrostatic precipitation, fabricfiltration, scrubbers, and mechanical collectors The electrostatic precipitator isthe most common process used for capturing fine ash particles in coal-firedutilities (11,22) ESPs capture ash by applying an electrical charge to the ashparticles The charged particles are subsequently attracted to oppositely chargedcollector surfaces in an intense electrical field Following collection, the particlesare sent to a hopper This technology is appropriate for capturing fly ash fromcoal with high sulfur content In fact, sulfur oxides in the flue gas may increasethe efficiency of particle capture in the ESP (10,11,22) The capability of ESP to

capture fly ash in the flue gas is more than 99% when this process is properlyoperated and maintained (10,11).

A fabric filter unit, or baghouse, is an appropriate technology for particulatecontrol in combustion processes that use coal with low sulfur content Thistechnology operates by forcing the flue gas through a fine mesh filter Fly ash istrapped and builds up on the filter surface The ash on the filter forms a cake,which is then periodically removed The efficiency of the filter increases as ashforms a thick layer on the filter surface However, thick cake formation also leads

to greater head losses in the process Fabric filters can remove over 99% of flyash from the flue gas in coal-fired utilities (10,11)

Scrubbers can also be used for particulate control and operate by applyingwater to contact the fly ash in the flue gas in a spray tower This technology alsocan remove over 99% of large ash particles, but less than 50% for particles withsize smaller than 1 or 2 µm (11,22) Mechanical collectors are instruments usedfor removing primarily large ash particles They operate by forcing the ashparticles against a collector wall, where the dry ash by-product is collected Theefficiency is lower than 90% for small particles

Desulfurization technology is used for capturing gaseous sulfur oxidesfrom flue gas in coal-fired utilities The use of desulfurization technology re-sults in the generation of FGD material There are two major types of FGD sys-tems: nonrecovery and recovery systems In the United States, 95% ofFGD systems are nonrecovery systems (11,22) Nonrecovery systems pro-duce by-product material, mainly calcium sulfate or sulfite, that has to bedisposed or used The nonrecovery FGD process can be separated into twotypes, wet and dry systems Wet systems operate by contacting the flue gaswith a slurry of water and sorbent Examples of wet scrubber systems includedirect lime, direct limestone, alkaline fly ash, and dual-alkali As mentionedearlier, approximately 90% of FGD systems in the United States use lime orlimestone as a sorbent (17) Typically, the calcium sulfite/sulfate sludge produced

in wet systems is dewatered and mixed with fly ash and lime to produce

“stabilized” FGD Examples of dry nonrecovery FGD systems include spraydrying and dry sorbent injection (11) Wet FGD systems produce more FGD perpound of coal than that of dry systems because of the use of water in the process.Recovery systems produce materials that can be used again in the FGD process,because the sorbent can be recycled Recovery FGD processes also have wet anddry systems Examples of recovery FGD systems include Wellman-Lord andmagnesium oxide systems and aluminum sorbent and activated-carbon sorbentsystems (11)

4.3 Types of Coal

Different types of coal have different heating values and also different ashcontents The highest-ranked coal with respect to heating value is anthracite,