8/3/2019 cfb6ash

1/7

A COMPARISON STUDY OF ACFB AND PCFB ASH

CHARACTERISTICS

K.M. Sellakumar and R. Conn

Foster Wheeler Development Corporation,

12 Peach Tree Hill Road, Livingston, NJ 07039, USA

A. Bland

Western Research Institute, 365 N. 9 th St, Laramie, WY, 82070, USA

6 th International Conference on Circulating Fluidized Beds

Wurzburg, Germany

August 22-27, 1999

8/3/2019 cfb6ash

2/7

A COMPARISON STUDY OF ACFB AND PCFB ASH CHARACTERISTICS

K.M. Sellakumar and R. Conn

Foster Wheeler Development Corporation, 12 Peach Tree Hill Road, Livingston, NJ 07039, USA

A. Bland

Western Research Institute, 365 N. 9 th St, Laramie, WY, 82070, USA

Abstract - The advent of fluidized bed combustion technology has provided avenuesfor environmental issues-free use of all types of fossil fuels. With the potentialcommercial application of Pressurized Circulating Fluidized Bed Combustion (PCFB)technology in the very near future, there is a need to understand the similarities anddifferences in the characteristics of solid by-products from the conventionalAtmospheric Circulating Fluidized Bed Combustion (ACFB) and the PCFB. Similar toACFB residues, the main components in PCFB residues are from:

- the inorganic constituents in the coal and incorporated sediments (Al, Fe, and Si),- sorbent derived elements (Ca, from limestone, or Ca and Mg if dolomite is used),

and- sulfur released from the coal during combustion that is captured by the sorbent.

However, the concentration of each component in the residues may show greatvariations, depending on the feeds and operating conditions in the unit. In general,the residues discharged from PCFB units with the same types of feeds and sulfurretention should have a relatively lower content of calcium but a higher content ofcoal-derived constituents than those from ACFB units, because a lower sorbent feedis required for pressurized systems. Also, the sorbent-derived components in theresidues from both systems are different due to the various sulfation mechanismsunder atmospheric and pressurized conditions.

Ash samples from the commercial ACFB plants and the Foster Wheeler 10 MWthPCFB pilot plant in Karhula have been used in this study. In this paper, the ashcharacteristics and where one type of ash- ACFB ash or PCFB ash has betterapplication over the other are described.

INTRODUCTIONCFB combustion has proven to be one of the most promising technologies for burning a wide rangeof coals and other fuels and handling wide variations in fuel quality, while still achieving strict airemission requirements. Low-grade fuels that have large ash content and very high sulfur do notnormally find acceptance in pulverized coal (p.c.) units. These fuels are burned efficiently in CFBsystems. However, if the sulfur content is large, then necessary sorbent has to be added to captureSO 2 in solid form.

CFB boilers generate two major waste streams, fly ash and bottom ash, which are a mixture of fuelash, unburned carbon residues, and lime particles coated with sulfate layers. The ash properties aresubstantially different from the p.c. ashes typically marketed as ASTM Class C and F fly ashes. Theoperating conditions for CFBC units, in addition to the fuel and sorbent characteristics, directlycontribute to the chemical characteristics of the ashes. CFB ashes generally contain a highercontent of calcium as an oxide and as a sulfate, but a lower content of silica and alumina than ashesgenerated from p.c. boilers. One notable exception is ashes resulting from the firing of low sulfuranthracites and bituminous coals; these by-products are composed primarily of fuel ashconstituents, since sorbent does not dominate ash chemistry. Consequently, the utilization optionsfor CFB ashes are somewhat more diverse than p.c. ash, due to the effect of sorbent on the overallash chemistry.

8/3/2019 cfb6ash

3/7

Pressurized fluidized bed combustion (PFBC) represents one of the most promising emerging CleanCoal Technologies (CCT). Circulating PFBC technology is being demonstrated at the pilot-scale atFoster Wheeler Energia Oy in Karhula, Finland. Western Research Institute (WRI) has completed athree-year project under sponsorship of the Electric Power Research Institute (EPRI), FosterWheeler Energy International, Inc., and the U.S. Department of Energy (DOE) Federal EnergyTechnology Center (FETC) that addressed ash use markets and options for PFBC technologies.FW supplied representative ash samples from the Karhula PCFB pilot combustor operation for thisstudy. The overall objectives of this study were to determine the market potential and the technicalfeasibility of using PFBC ash in high-volume use applications.

The Environmental Protection Agency (EPA) made a final rule that effective September 2, 1993,four large-volume fossil fuel combustion (FFC) waste streams from electric utility power plants areexempted from RCRA (Resource Conservation and Recovery Act of 1976) Subtitle C for hazardouswaste regulations (Federal Register 1993). The waste streams include fly ash, bottom ash, boilerslags, and flue gas desulfurization (FGD) sludge. Nevertheless, the fluidized bed combustion (FBC)waste streams were not included in this final regulatory determination due to the lack of information.Since early 1990s, extensive studies have been conducted on the ACFB and PCFB ashes (Young,1996; Conn and Sellakumar, 1997, Conn, Wu, and Sellakumar 1997; Bland, 1998).

In this paper, a review of the ACFB and PPCFB processes, key changes in the by-productconstituents, and attendant differences in the physical and chemical properties are described. Inaddition, the current and potential uses of the by-products are outlined.

ACFB AND PCFB PROCESSESIn fluidized bed combustors, limestone is added in the bed for sulfur capture. Probable sulfurcapture mechanisms in the ACFB and PCFB combustors have been summarized earlier (Koskinenet al., 1993). Numerous studies have confirmed that at atmospheric conditions, the first step in thesulfur capture process is the calcination of CaCO 3.

CaCO 3 (s) CaO (s) + CO 2 (g) (1)The calcination reaction proceeds significantly faster than the next step that is the sulfur capturestep, called the sulfation reaction.

CaO (s) + SO 2 (g) + 1/2 O 2 (g) CaSO 4 (s) (2)

Under pressurized fluidized bed combustion conditions, the high partial pressure of CO 2 that existsby virtue of the high combustor pressure, prevents the decomposition of CaCO 3. At high O 2 partialpressures, the desulfurization reaction in pressurized conditions is via the "direct" sulfation ofCaCO 3.

CaCO 3(s) + SO 2(g) + 1/2 O 2(g) CaSO 4(s) + CO 2(g) (3)However, in the presence of some catalysts SO 2(g) can be converted to SO 3(g), and the followingreaction is also possible (Hajaligol et al. 1988):

CaCO 3(s) + SO 3(g) CaSO 4 (s) + CO 2(g) (3a)

Thermal decomposition of CaSO 4 in normal pressurized fluidized bed combustion conditions is notprobable because of low temperatures and high pressures in the reactor. However, according toLygnfelt and Leckner (1989) significant amounts of SO 2 may be released from the sorbent in thepressurized fluidized bed combustion if conditions become reducing in the reactor.

In summary, it can be concluded that under PCFB conditions:- the sulfation of sorbents takes place by a direct reaction between CaCO 3 and SO x..- the sulfation rate increases with temperature and- the total pressure in the combustor does not limit the sulfur capture by the sorbent.- The surface structure of the ash particles from the PCFB process is likely to be different from

that of the ash from the ACFB process.

8/3/2019 cfb6ash

4/7

CHARCTERIZATION OF ACFB ASHChemical Properties: Key oxides of interest for ash use in typical ACFB bottom ash and fly ashare presented in Table 1. The chemical characterization testing included major elementcomposition, as well as phase analysis. There is a general decrease in CaO and SO 3 withdecreasing sulfur content of the fuel burned. Fuel ash contributes silica, alumina, and certainamounts of alkalis. The fly ash and the bed ash were also analyzed by X-ray diffraction (XRD) forphase composition. The data confirm the observation that the ashes are composed principally ofanhydrite[CaSO 4], lime [CaO], quartz [SiO 2], and associated oxides of iron, magnesium, anddehydroxylated clays originating from the fuel ash components.

Physical Properties: The general physical properties of the ashes were also determined, includingpoured and packed bulk densities, specific gravity, particle size distribution, and moisture. The flyashes all were relatively fine with greater than 80% passing a 200-mesh screen (74 m). As aresult, these ashes can readily be made into cement-type pastes without further milling. The pouredbulk density of the fly ashes ranged from about 540 (low S fuel) to 916 (high S fuel) kg/m 3; thecompacted bulk density of the fly ashes were slightly higher and ranged from 840 (low S) to1167(high S) kg/m 3. The specific gravity ranged from 2.2 (low S) to 2.7 (high S) for the fly ashes.

CHARACTERIZATION OF PCFB ASHThe study of PFBC ash use options has included two different ashes: ash from the combustion oflow-sulfur Powder River Basin subbituminous coal (Black Thunder) with limestone sorbent and thecombustion of high-sulfur Illinois Basin coal with a limestone sorbent. (Bland, 1998).

General Chemistry: With the exception of relatively high mineral carbon, the chemistry of thePCFB ashes is typical of ashes from ACFB of low-sulfur and high-sulfur coals using limestone anddolomite sorbents. Phase analyses of the ashes by X-ray diffraction show that the PCFB ashes arecomposed principally of anhydrite (CaSO 4 ), calcite (CaCO 3 ), coal ash oxides, and dehydroxylatedclays. The lack of lime (CaO) in the PFBC ashes is distinctly different from AFBC ashes, whichcontain large amounts of lime. As stated earlier, in PFBC systems, the partial pressure of CO 2favors both calcination and recarbonization. This results in low lime and high carbonates (calcite) inpressurized FBC ash. Key oxides of interest for ash use are presented in Table 1. The loss onignition (LOI) is composed of the moisture and the organic carbon. The LOI in the PFBC ashes has

been corrected for mineral carbon. Moistures are less than 0.1% and the organic carbon contentsare less than 2%. The free lime (CaO) content of the PFBC ashes was determined by ASTM C-25to be in the range of 0.5 to 1.0%. The majority of the lime appears to still be carbonated in the formof CaCO 3.

Physical Properties : The general physical properties of the ashes were also determined, includingbulk densities, specific gravity, and particle size distribution. The PCFB fly ashes measured poured-bulk densities of 795 to 948 kg/m3 and specific gravities of 2.73 to 2.34 for high sulfur and low sulfurfuels respectively. This is just opposite of the ACFB fly ash characteristics.

DISCUSSIONAsh Characteristics: Leachate characteristics of the ashes were tested according to the U.S. EPA

Toxicity Characteristics Leaching Procedure (TCLP) (EPA CFR Part 241). The ash leachate datasubstantiate that none of the leachates generated from the ACFB and PFBC ashes exceeded the1976 RCRA limits. As such, these ashes would not be classified as hazardous. Ashes from othercoal-fired power systems are already categorized as nonhazardous and have been given anexclusion from these RCRA requirements (Table 2).

Ash Utilization Studies: Diverse utilization options have been studied for ACFB and PCFB coalashes. The potential applications include:- construction applications: cement substitute, concrete block production, brick production, soil

stabilizer, roadbase/subbase materials, structural fill materials, andsynthetic aggregates;

8/3/2019 cfb6ash

5/7

- agricultural applications: liming and soil amendment;- waste stabilization: acidic waste stabilizer and sludge stabilizer.

The key fuel and sorbent properties, which can influence ash characteristics, are sulfur content, ashcontent, and ash composition including the form of Ca (CaCO 3 or CaO). Other important fuelproperties influencing ash characteristics are the size and friability of the fuel minerals. Theseproperties will impact on how these minerals will exit the CFB in the fly or bottom ash stream, whichcan have a significant effect on the utilization of these streams. Extensive literature is now availableon the ACFB ash use options based on the studies since early 1980s. (Anthony et al. 1995, Kilgouret al, 1991, Sun et al. 1980, Tavoulareas et al. 1987; Conn et al. 1997; Bland, 1998; Bland, 1999).Details on PCFB ash use has been reported since 1993 (Bland et al. 1994; Bland, 1998). Asummary of the current know-how is given below.

Ash Use in Construction Applications: ACFB and PCFB fly ashes cannot be classified as ClassF or C, because of low FAS (ferric oxide, alumina, and silica) and high SO 3 content (Table 1). Eventhough CFB fly ash may not qualify as a Portland cement admixture, it may have the potential foruse in concrete blocks. Bottom and fly ash can be used as an aggregate and pozzolan in concreteblocks. The bottom ash used as an aggregate has a lower unit weight than many naturallyoccurring aggregates, thus reducing the weight of the block. CFB fly ash with properties of Class Cand F can be used as a partial replacement for Portland cement in some block plants. Generally,the carbon content of the fly ash must be limited to less than 5%, since segregation can occurduring handling and result in nonuniform blocks.

The free lime and sulfate contents of CFB ashes can limit their utilization in concrete blockproduction. Free lime present in ash will form water-soluble calcium hydroxide, resulting inweakening of the block from contact with moisture. As with Portland cement concrete, ettringite canform in concrete blocks due to high ash sulfate levels resulting in mechanical weakening of theblock. It is possible that CFB ash may replace some Class C or F fly ash or Portland cement inmore moderate strength blocks. Such materials may not be preferred for heavy constructionapplications, but more for residential uses (Conn et al. 1997). Testing of PCFB ash has indicatedthat PFBC ash, when mixed with low amounts of lime, develops high strengths, suitable for soilstabilization applications and synthetic aggregate production. Synthetic aggregate produced fromPFBC ash is capable of meeting ASTM/AASHTO specifications for many construction applications(Bland 1998).

Soil/Mine Spoil Amendment: The technical feasibility of ACFB/PFBC ash as a soil amendmentwas examined for acidic problem soils and spoils encountered in agricultural and reclamationapplications. The results of the technical feasibility testing indicated the following:

Ash streams from CFB boilers firing low sulfur semi-anthracite and anthracite waste would notbe good candidates for agricultural liming due to very low free lime ( and CaCO 3 equivalent)contents. On the other hand, ash streams from CFB boilers firing bituminous coals may besuitable for liming, depending upon how calcium is partitioned between the fly ash and bottomash.

PFBC fly ashes were effective acid spoil and sodic spoil amendments though they have lowCaO content. In a comparison with ag-lime, the fly ashes reacted with the acidic spoil at aslower rate and the final pH of the treated material was slightly lower (i.e., fly ash treated, pH 7and the ag-lime treated 8).

the greenhouse studies demonstrated that PFBC fly ash amended spoils resulted in higher plantproductivity than the ag-lime-amended spoils. These results possibly are due to pH andnutritional issues, but root penetration was undoubtedly a factor.

CFB ash streams can also be used to stabilize waste streams from a variety of processingoperations. This stabilization includes solidification and fixation of sludge materials for landfilling,

8/3/2019 cfb6ash

6/7

neutralization of acidic wastes, and municipal sludge waste sludge. For each of these applications,the suitability of CFB ash is enhanced by its free lime content

CONCLUSIONIn-house and literature data show that ash streams from CFB boilers firing diverse fuels have thepotential for use in one or more applications. FW has studied the ACFB bottom ash and fly ashcharacteristics from both an environmental impact and by-product utilization standpoint. First, therisk screening criteria and exposure analysis results indicate that these CFB wastes are as clean orbetter than those generated from conventional combustors such as p.c. boilers. As a result, CFBby-products can be used in various applications without impacting the environment. The exactutilization options for ACFB by-products will depend primarily on the type of fuel being fired, and to alesser extent the type of sorbent utilized for sulfur capture. The PCFB ash differs from the ACFB inthe uncalcined CaCO 3 in the by-product.

As with ACFB ashes, there is a significant market potential for PFBC ash in the construction and soilamendment industries. In particular, PFBC ash represents a technically viable material for use inthese currently established applications for conventional coal combustion ashes. It is possible tomodify the hydration reaction chemistry of the PFBC ashes through such processes as limeenhancement to produce the geotechnical properties required for construction applications. As aresult, PFBC ash should be viewed as a valuable resource, and commercial opportunities for thesematerials should be explored for future PFBC installations.

REFERENCESAnthony, E.J., Iribane, A.P., and Iribane, J.V., Proc. of the 13th Intl. Conf. on Fluidized Bed

Combustion, Orlando, FL, (1995), Vol. 1, pp. 523-533.Bland, A.E. US DOE Contract DE-FC21-93MC30126, Final Report (Western Research Institute

Report WRI-98-R017), June, (1998).Bland, A.E., Proc ., 1998 Advanced Coal-Based Power and Environmental Systems 98 Conference,

US Department of Energy, Morgantown, WV, (1998).Bland, A.E., Proc. of the 15 thIntl. Conf. on Fluidized Bed Combustion, Savannah, GA, (1999).Conn, R.E., and Sellakumar , K.M., Proc. of the 14 th Intl. Conf. on Fluidized Bed Combustion,

Vancouver, Canada, (1997), pp. 507-518Conn, R.E., Wu, S., and Sellakumar, K.M., 1997 Pittsburgh Coal Conf., Taiyuan, China (1997).Conn, R.E., Sellakumar , K.M. and Bland, A.E., Proc. of the 15 th Intl. Conf. on Fluidized Bed

Combustion, Savannah, GA, (1999).Federal Register, Part V, United States Environmental Protection Agency, 40 CFR Part 261, Vol.

58, No. 151, August 9, 1993.Hajaligol, M.R., Longwell, P.J., Sarofim, A.F, Ind. Eng. Chem. Res. 27, (1988), pp.2203-2210.Kilgour, C. L., and McGowan, K.I., Electric Power Research Center, Iowa State University, Final

Report, October (1991).Koskinen, J., Lehtonen, P., and Sellakumar , K.M., Proc. of the 13 th Intl. Conf. on Fluidized Bed

Combustion, Orlando, FL, (1995), pp. 369-378Lygnfelt, A., and Leckner, B., Proc., 10th International Conference on Fluidized Bed Combustion,

Manaker, A., ed., ASME, New York, (1989), pp. 675-684Sun, C. C., and Peterson, C. H., Proc. Sixth Intl. Conf. on Fluidized Bed Combustion, Philadelphia,

PA, (1980), Vol. 3, pp. 900-912.Tavoulareas, S., Howe, W., Golden, D., and Eklund, G., Proc. of the 9 th Intl. Conf. on Fluidized Bed

Combustion, (1987), Vol. 2, pp. 916-926.Young, L., Fifth Intl. Conf. on Circulating Fluidized Beds, Beijing, (1996), Pr5

8/3/2019 cfb6ash

7/7

Table 1 Typical Ash Chemistry of Fly Ashes from Different Fuels

Fuel ACFB-High

sulfur Bit.

ACFB -Low

sulfur Bit.

PCFB-High

sulfur Bit.

PCFB - Lowsulfur Bit.

Ash % 12.19 7.32 11.3 7.9

S, % 4.60 0.55

Pozzolan in PortlandCement

Class3.39 0.48

Ash, wt. % FA (1) FA F C FA FA

SO 3 7.4 8.0 5 max 5 max 20.83 12.27

FAS (3) 68.8 49.2 70 min 50 min 50.63 57.57

Free CaO 13.7 28.5