Trace element speciation in fluidized bed coal combustion

by-productsPetr SulovskýDept. of Mineralogy, Petrology & GeochemistryMasaryk UniversityBrnoCzech Republic

or:The Fly Ash Story

• Parents:• poor Brown

Coal• shiny Pyrite

Children:

• Stinky Flue Gas

•Solid Deposition

•Fly Ash

Step son:

Acid Rain

Environmental impacts of coalcombustion

• Thermal polution• CO2 emissions Greenhouse effect• Sulphur dioxide and nitrogen oxide

emissions acid rain• Emission of potentialy hazardous trace

elements• Large volumes of coal combustion residues

Mineral composition of North Bohemian lignitous coals

• Organic matter• Minerals:

– Major - quartz, kaolinite, illite, calcite, pyrite, marcasite, gypsum, smectites

– Accessory – siderite, apatite, galena, feldspars, micas, halite, barite, crandallite group, zeolites

Iron disulfide forms

North Bohemian brown coal -chemical composition

• Water content 20 – 30 %

• Incombustible components 5 – 25 (30) %

• Combustible components 50 - 60 %

• Calorific value 14 – 20 MJ/kg

Factors controlling the acid-forming potential of coal

• Sulfide features: abundance, type (syngenetic, epigenetic), crystallinity, morphology and grainsize

• Specific surface of the pulverised coal• Inherent neutralisation potential of the coal

(carbonate content)• Oxygen availability• Activity of sulphide consuming bacteria

(Thiobacillus ferooxidans)

Fluidized Bed Combustion

• Fine particle mix of fly ash and lime • Burned at 750-8500C • Furnace retention time 20 minutes• No slag production• Reduces SO2 emissions by 95%• Less volatilisation

FBC boiler scheme

Fluidized Bed Combustion Reactions

4FeS2 + 9 O2 2Fe2O3 + 6SO22SO2 + O2 2SO3

SO3 + CaCO3 CaSO4 + CO2SO2 + CaO CaSO3

2SO2 + 2CaO + H2O 2CaSO3.H2OCaSO4 + H2O CaSO4.2H2O

2K0,65Al2[Al0,65Si3,35]O10(OH)2 (illite)2H2O + K2O.3Al2O3.6SiO2 (metaillite)

Conventional coal combustion

Conventional coal combustion

Main differences between classicaland fluidised bed combustion

Factor or process

C lassical com bustion FB C

U sual com bustion tem peratu re

1400 - 1500 oC 820 - 850 oC

C oal grain heating rate 1 oC . s-1 1 .000 oC . s-1

Furnace reten tion

tim e seconds > 20 m inu tes

Inpu t m aterials coal coal +

additive

Main differences between classical andfluidised bed combustion (cont´d)

Factor or process Classical FBC

Glass formation massive scarce

VolatilizationHg, Se, Pb, ZnO, Ba(OH)2, As2O3

etc none

Ash grain forms globular, rounded irregular, rather

sharp-edged

Grain size < original

(thermal shock fragmentation)

original

Main differences between classical andfluidised bed combustion (cont´d)

Factor or process Classical FBC

Neoformation of mineral phases

extensive, many phases /

extensive, few phases

(anhydrite, meta-clays)

Changes in PHE speciation distinct limited

Concentrations of volatile elements

lower (major portion escapes with flue gas)

higher (up to 95% of the

amount present in coal)

Surface enrichment pronounced, high visible, lower

Main differences between classical andfluidised bed combustion (cont´d)

Factor or process Classical FBC

Thermal metamorphism strong weaker

Recrystallisation abundant scarce

Mineral neoformation

massive, many species

massive, limited to a few species

Change of trace element

bonding massive limited

Mineralogy of classical vs. FBC ashes

Combustion method Classical FBC

Main phases

glass, mullite, anortite, quartz,

cristoballite, magnetite and other spinels,

gehlenite

quartz, metaillite, metakaolinite,

anhydrite, periclase, lime,

portlandite, hematite

Pyrite decomposition

products magnetite haematite,

maghemite

Forms of occurrence of elementsin fly ash

Depend on• Chemical form of the element in coal• Influences during coal cleaning, conversion,

leaching, weatheringControl:• Impacts on the environment, technological

behaviour, possibility of ash usage as secondary raw material

Concentrations of some trace elements in North Bohemian coal and in solid residues

after conventional coal combustionConcentration< 10-6 Ag, Bi, Ga, Ge, Mo, Sb, Tl, W

Concentration10-5 - 10-6

Be, Ga (sub-micron fly ash), Ge (sub-micronfly ash), Pb (slag), Sb (sub-micron fly ash), Sn(slag and sub-micron fly ash)

Concentration10-4 - 10-5

As, Co, Cu (fly ash), Mn (slag and sub-micronfly ash), Ni, Pb (fly ash and sub-micron flyash), Sn (fly ash), Sr, Zn

Concentration10-3 - 10-2

As (sub-micron fly ash), B, Cr, Cu (slag andsub-micron fly ash), Mn (fly ash), V, Zn (sub-micron fly ash)

Besides the above, coal or fly ash often contain increased levels of Cd, Cl, F (in N-Bohemian brown coal hundreds of ppm) , Hg, Se.

Specific features of classical fly ashes from North Bohemian brown coal

• Uniform composition of glass matrix• Absence of Ca-rich glass known from Low-Saxony

brown coal combustion residues (Enders 1994).• Considerable portion of Ca and Mg occurs as

spherical di-calcium ferrite or Mg–ferrite particles.• The surface enrichment model (Natusch, Wallace

1974) is of limited validity here.• The content of the volatile elements (As, Pb, Zn) is

lower than in fluidized bed ashes ¨the difference escaping with flue gas into the environment.

• More than 70% of all studied PHE-s are bonded to insoluble phases of coal combustion residues

Classical coalcombustion

fly ash

Glass composition

Classical coal combustion fly ashCa-ferrite/

Mg-ferrite

Glass w. mullite /

magnetite

magnetite/ Al-spinel / glass

spinels / glass

Specific features of FBC ashes from North Bohemian brown coal

•The most abundant crystalline phase is anhydrite (17 -30%, some > 45%). It bonds only some Ni and Ba.•Calcium sulphate aggregates show in most cases atomic ratio Ca/S ~ 2 - 3 : 1. The Ca excess decreases the leachability of Ni, Zn and As.• Thermally metamorphosed clay minerals and micas bond most of Cr, V, and Ba content. •The highest affinity to Fe oxides show V, As, Co & Mn. •Exposed to outdoor conditions, FBC ashes can change their composition (due to residual CaO/MgO).

Surface enrichment vs. separate phases

Classicalcombustion

FB combustion

FBC ashes

Bottom ash

Textile-filter captured ash

Reactions occurring in CCR products• Hydration of and/or carbonation of unspent CaO and

MgO (large increase in volume)• Formation of ettringite from alkaline solutions

containing residual lime, free Al from meta-clays, and sulphate ions:

6Ca2+ + 2Al(OH)4- + 3(SO4)2- + 4(OH) →Ca6Al2(SO4)3(OH)12 . 26H2O

• In CCR rich in free silica can form another member ofthe ettringite group – thaumasite:

6Ca2+ + 2H2(SiO4) + 3(SO4)2- + 2CO2 + 12(OH)-

Ca6Si2(SO4)3(CO3)2(OH)12 . 24H2O• other products forming from activated aluminosilicates:

- monosulphoaluminate- strätlingite (hydrated gehlenite – Ca2Al2SiO7.8H2O)

Volume changes due to unspent MgO / CaO hydration

FBC ashes reprocessing technology

• Must allow that all CCR are utilized

• Should require minimum preprocessing

• Should support fixation and retention of hazardous trace metals over time.

Possibilities of controlling the bonds of hazardous elements in CCR products

•hydrothermal alteration - production of zeolite or light-weight concrete)

• vitrification, mineral wool production

Possibilities of controlling the bonds of hazardous elements in CCR products

• solidification and addition to cold-worked building materials (concrete, mortar)

• pelletisation, agglomerate production

Agglomerate production

• Simple: blending of individual CCR streamswith water pelletization.

• Economical: only mechanical kneading ofash paste.

• „Leftover-free“: bottom ash, baghouse filter ash & cyclone-trapped ash are blended in the same proportions as they come out from the FBC facility.

• Potential for hazardous elements fixation• Various product uses

Phase composition of agglomerates made of FBC ashes

• residual FBC ash phases (quartz, thermally modified clay minerals, portlandite, anhydrite etc.)

• newly formed phases - ettringite (C6AS3H32), monosulphate (C4ASH12), thaumassite, gypsum, strätlingite (C2ASH8).

• The amount of ettringite formed in agglomerate depends not only on available free lime, sulphateand water content in the paste, but probably also on the volume of free pores in the pellets

Features of FBC-ash based pellets

Due to differing granularity of bottom and fly ash, the pellets are zoned. The core of the pellet is usually made up by a larger grain of portlandite, anhydrite, or other residual phase, enveloped by rusty rim zone composed of small grains of filter ash with increasedhaematite content.

The forms of PHE occurrence in agglomerates

• positive relation between certain heavy metalsand Fe oxide (hematite)

• association of F with anhydrite (maximum observed F content achieving 0,9%).

• correlation between Ba and K (observed also in CCRs and in coal; K-feldspar derived)

• correlation TiO2 - V, Ba & Pb, occurring also in CCRs

The relation between anhydrite content and mobile + exchangeable PHEs:

• Strongest in vanadium (r=0,55)

• Medium affiliation (significance level between 90 and 95%) - Co, Zr and Ba

• Ni (occurring in coal together with Co)shows only weak correlation to anhydrite (r = 0,27)

Additive - derived alkaline substancesand PHE speciation

• The preservation of calcite or its neo-formation can support fixation of Co, As and V (r = -0,21, -0,19, -0,29). There is no such relation in Cr, Ni, or Cu.

• Residual portlandite or periclase content restricts the leachability of all studied PHEs, esp. Co, Ni, Cu, Cr, and V: r = -0,49 to -0,43 (statistically significant at 90 - 95% level). Less obvious is the effect with As, Ba a Zn (r = -0,39 až -0,30).

• These observations are in accordance with results of de Groot et al. (1989), who observed enhanced fixation of these elements at high pH (in case of FBC by-products caused by high free lime).

Ettringite formation in agglomerates

Ettringite formation in agglomerates can generally be seen as a beneficial phenomenon:

• The enhanced ettringite formation has been found to have potential for the fixation and stabilization of hazardous wastes (Hassettet al. 1992): Ettringite admits into its crystal lattice certain anions (borate, selenite, maybe also chromate), as well as cations occupying both Ca2+

sites (Cu, Co, Ni), and Al3+ sites (Fe3+, probably also Cr3+).

• It increases the mechanical strength of the product (by forming the 3-D network of ettringiteneedles)

Ettringite aggregate in agglomerate

The relation between ettringite contentand mobile + exchangeable PHEs:

• Strongest in Cr (r = 0,54) – can substitute Al (more probable than CrO4 SO4 – Hassett, pers.comm.).

• No relation between the weakly fixed As and V fractions and ettringite content (rAs = 0,02, rV = 0,04).

• Weakly fixed portion of Cu, Ni and Co correlates positively with ettringite content, although their relationship is not very close (r = 0,16 - 0,36).

• Se and B were not present in increased levels; experiments of Hassett et al. (1993) have shown that high amounts of these elements can be incorporated into ettringite.

Ettringite content controls

• Besides the chemical and phase composition ofCCR, the content of ettringite in agglomerates is given by the amount of kinetic energy supplied by the pelletizer.

• On ageing, the formation or decomposition of ettringite is above all controlled by humidity of the surrounding environment.

Conclusions• Combustion of North Bohemian brown coal for

power generation represents a threat to theenvironment

• Change from classical- to fluidised-bed coalcombustion requires to seek methods for safe re-use or disposal of FBC residues

• Of possible CCR conversion technologies, agglomeration appears to be very economical

• The highly alkaline nature of FBC ashes restricts theleachability of most heavy metals

• Ettringite formation in agglomerates enhances thefixation of some potentially hazardous elements, above all Cr, Cu, Co, Ni, Se and B