formation of strong primary acidity in combustion particulate: causes and remedies

*Corresponding author.

Atmospheric Environment 33 (1999) 3551}3558

Formation of strong primary acidity in combustion particulate:causes and remedies

P. Primerano, I. Campisi, S. Di Pasquale, F. Corigliano*

Dip. Chimica Industriale, Universita% di Messina, 98166 Messina, Italy

Received 28 August 1998; received in revised form 18 January 1999; accepted 3 February 1999

Abstract

Having observed great variations in the strong primary acidity (SPA) content of the ash of fossil fuels with similarsulphur contents, we studied the chemical causes of the formation of SPA in ashes. The main factors behind thesevariations were found to be: (a) combustion conditions and the extent to which they favour or do not favour theformation of SO

3(excess of oxygen, presence of suitable catalysts, etc.); (b) the amount of basic oxides present in the

mineral fraction of the fuel in relation to the SO3

produced; and (c) the basic strength of these oxides. The variable but,nevertheless, high SPA contents common only to oil ashes can be explained in the light of the modest values for factors (b)and (c) and the conditions favourable to factor (a) in this fuel. Remedies involve the in-#ame addition of highly basicoxides in order to arti"cially increase factors (b) and (c). ( 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Ash acidity formation; Atmospheric acidity; Oil #y ash; Sulphur in fuels

1. Introduction

The current legal limitations on industrial emission ofparticulate do not contemplate its reactivity or danger-ousness to the environment but are only designed to keepits concentration in fumes (CL"50 mg m~3) (EEC Di-rective, 1988; EPA CFR N. 40; Italian DPR, 1988) belowthe smog threshold.

According to regulations the particulate produced incoal-"red power stations (whose concentration in fumesis +103 CL) must almost all be "ltered (*99.8%)whereas it may be emitted un"ltered in oil-"red powerstations (concentration +CL).

In a previous study we drew attention to the fact thatashes produced in oil-"red power stations are potentialcarriers of strong primary acidity in the atmosphere sincethe acidity conveyed by these ashes is already strong andhighly reactive on emission. Indeed, we showed thatstrong primary acidity is transmitted almost immediately

on contact with water, to which it confers very acid pHvalues (0}2) and it transmits considerable concentrationsof sulphur (VI), polyvalent metals (especially vanadiumand iron) and very high total acidity (2}3 mmol g~1)(Primerano et al., 1998).

On the other hand, other types of particulate emittedinto the atmosphere by natural and arti"cial activities areusually neutral or even alkaline and therefore contributeto an increase in smog but not in atmospheric acidity.Ashes produced by the burning of coal in particular arean example of particulate with these characteristics (Bil-linge et al., 1990; Puccio, 1983). It follows from this thatsince particulates have di!ering characteristics, it is notsu$cient to monitor overall concentrations as currentlegislation requires.

We thought it important to "rst examine in depth thereasons for such variation in the acidity of the solidresidues of combustion despite their originating fromfuels containing comparable proportions of sulphur andwe tried to identify a correlation between the acidity ofthe ashes and the composition of the fuel, especially interms of sulphur and other mineral fraction concentra-tions, in order to explore the chemical causes of the

1352-2310/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 1 1 - 9

Table 1Composition (mean values of six replicates) of ash samples (wt% of the dried sample)

Sample Unburnt C V2O

5#V

2O

4! Fe

2O

3NiO MgO SO

3

SFM-85 59.9 13.78 0.84 1.27 3.68 18.00TI-81 57.0 5.59 7.41 1.43 0.16 22.95SFM-83 56.2 1.94 14.12 1.33 0.59 18.47TI-94 62.7 3.72 1.92 1.33 0.98 20.07PG-94 64.0 7.03 3.78 2.04 0.35 19.02

!Calculated as V2O

5.

formation and presence of strong primary acidity in fuelashes. Account has been taken of previous studies intohow the components of mineral matter are transformedduring combustion, as described for example by Hu!manet al. (1991a,b), Huggins and Hau!man (1997) andTaghiei et al. (1991) for coal, by Aurisicchio et al. (1995),Burriesci et al. (1984), EPRI (1983) and Primerano et al.(1998) for oil and by Henry et al. (1980) and Homolyaand Fortune (1978) for both. Our method involves theformulation of a likely chemical hypothesis and veri"ca-tion of its validity by checking agreement between thedata calculated according to the hypothesis and experi-mental data.

Once causes are known, a further objective of thiswork is to identify and propose possible remedies of botha technical and legislative nature.

2. Experimental part

A treatment procedure designed to provide answers toour objectives was applied to the data found byPrimerano et al. (1998). While the experimental proced-ures employed and the data obtained are thoroughlydescribed there, those essential to understanding thetreatment to be applied will be re-presented here for thesake of convenience.

2.1. Samples

The ash samples used were sampled at various times(1981, 1983, 1985, 1994) from the hoppers of electrostaticprecipitators (ESP) at Sicilian oil-"red power stationsin San Filippo del Mela (SFM-83, SFM-85), Messina;Termini Imerese (TI-81, TI-94), Palermo and PrioloGargallo (PG-94), Syracuse. Care was taken over theconservation of all the samples in order to ensure thatthere would be no change in their composition betweenthe time of collection and today. Periodical checks werecarried out.

2.2. Sample leaching

(a) One gram aliquots of the ash samples were totallydissolved in a 25 ml mix of concentrated H

2SO

4,

HNO3

and HClO4

(with a ratio of 3.5 : 3.4 : 1.0) byheating it until total oxidation of the carbonaceousfraction was achieved. After the removal of excessHNO

3and HClO

4by heating, the solution was

cooled, "ltered (sometimes a white deposit of hy-drated silica was obtained), diluted to 100 ml andanalysed for vanadium, iron, nickel and magnesium(Burriesci et al., 1984). In the mix of concentratedacids H

2SO

4was omitted when sulphur had to be

determined.(b) Twenty "ve grams aliquots of dried powder were put

into a glass column with a porous septum and leachedby percolating deionized water through them. Threesubsequent 25 ml fractions of extract were collected ina multistage mode and one 125 ml fraction ina single-stage mode.

2.3. Analysis of samples and leaches

Unburnt carbon. An aliquot of dry sample was weighedand then leached with water, dried, reweighed, burnt at6003C and weighed again. The di!erence between the lasttwo weighings divided by the weight of the sample gavethe unburnt carbon content shown in Table 1.

Vanadium (V) was determined in the solutions resultingfrom leachings (a) and (b) by potentiometric redox titra-tion (Pt electrode) in a 5% sulphuric acid medium witha standard iron (II) solution prepared by weighing pureMohr salt (Burriesci et al., 1984; Rulfs et al., 1956).

Vanadium (IV#V) was determined in leach (b) by thesame titration preceded by the oxidation of vanadium(IV) to the 5` oxidation state with KMnO

4. Excess

KMnO4

was eliminated with solid KNO2

and excessKNO

2with solid urea (Burriesci et al., 1984; Rulfs et al.,

1956).Total iron, nickel and magnesium were determined in

the leaches by means of atomic absorption in an air}acetylene #ame.

Iron (II). Ferricyanide and 1,10-phenanthroline testswere used to reveal the presence of iron (II) species in theaqueous (b) leaches (Charlot, 1963).

Sulphur was gravimetrically determined as BaSO4

inthe solution resulting from modi"ed leaching (a) andleaching (b). Determination was preceded by the

3552 P. Primerano et al. / Atmospheric Environment 33 (1999) 3551}3558

separation of the leached cations on resin. To this end theaqueous leaches were percolated through a Dowex50}100 mesh cationic resin in H` form to obtain colour-less eluates where sulphur (VI) was then determined(Koltho! and Sandell, 1956; Samuelson, 1963).

Sulphur (IV). A potassium permanganate test (Charlot,1963) was used to reveal the presence of sulphur (IV) speciesin the aqueous leaches after removal of cations as before.

pH was measured by a combined glass electrode calib-rated with standard bu!ers having an ionic strengthcomparable to that of the samples (estimated at ca.1 kmol m~3). In the pH range from 0 to 0.5, the standard-ization of the electrode was e!ected every 0.1 pH units; inthe range 0.5}1, every 0.25 pH units.

Total acidity was titrated in the aqueous leaches witha 0.1}0.5 kmol m~3 standard NaOH solution until a pHof 5.6 } considered representative of environmental neu-trality } was reached.

3. Results and discussion

The chemical composition of ash as regards maincomponents is set out in Table 1.

We can observe: (a) a prevalence (from half to two-thirds in weight) of unburnt carbon (deriving from in-complete combustion of the heavier fractions) into whichthe ashes, in the strict sense, are incorporated; (b) thepresence of a single acid oxide } sulphur oxide } butsurprisingly only at 6` oxidation state; and (c) amongbasic oxides, a surprising presence of vanadium (IV)rather than (or together with) vanadium (V) and anexcessively high percentage (in at least two or threesamples) of iron if related to the iron content of oil. Asregards the sulphur, most of the sulphur content of fuel(over 95%) is certainly oxidized to SO

2: while ack-

nowledging the customary use of an excess of air incombustion, it is di$cult to believe that more than a fewpercentage units of sulphur (VI) might be formed withrespect to the total sulphur in the fuel and therefore wewould have expected more sulphur (IV) than (VI) in theashes. The presence of only sulphur (VI) in the ashes, andin such high percentages, needs to be explained. As re-gards vanadium, the stable species in combustion condi-tions is certainly V

2O

5(Aurisicchio et al., 1995; Robie et

al., 1978), which makes it reasonable to assume that somechange occurs during the cooling phase of the fumes whichwill require explanation. As regards the sometimes excess-ively high percentages of iron, these could conceivablyderive from plant corrosion, but the absence of iron (II) isa surprise, at least in part, and also requires explanation.

3.1. Composition of their aqueous extracts

As previously stated (Primerano et al., 1998), theseashes, when contacted with water, immediately transfertheir strong primary acidity content to it, conferring pH

values ranging from 0.25 to 2.39. Comparison of free(deduced from pH) and total acidity (TA, determined byalkalimetric titration up to pH 5.6) values showed thatthe latter are usually much higher than the former. Thehypothesis advanced is that species other than sulphurcontribute to the total acidity, hydrolyzing in an acidmedium and producing acidity. The components that aresolubilized from ashes include: vanadium (IV), iron (III),sulphur (VI), nickel (II), magnesium (II) and other minorcomponents. Vanadium (IV) and iron (III) can give riseto these hydrolytic reactions in an acid environment. Wetherefore determined their content in each sample and ineach aqueous extract (obtained by extraction with waterin one or more stages) and compared this with total (TA)and free (pH) acidity content. Results are set out inTables 2 and 3.

3.2. Species responsible for strong primary acidity in oil yyash: model and experimental validation

The results show that the total acidity is proportional tothe vanadium (IV) and iron (III) content of the leachedsolutions. To explain these results it is necessary to under-stand the equilibria that can take place in the aqueousextracts between the species transferred from the ashes.Such equilibria should involve the dissociation of sulphu-ric acid and its only partially neutralized forms (free orsulphuric acidity) and the hydrolysis of iron (III) andvanadium (IV) (nickel (II) does not hydrolyse appreciablyin an acid environment) (Sillen and Martell, 1964):

H2SO

4PH`#HSO~

4

HSO~4HH`#SO2~

4

dissociation of sulphuric acid and bisulphate ion (1)

Fe3`#H2OHFe(OH)2`#H`

Fe(OH)2`#H2OHFe(OH)`

2#H`

Fe(OH)`2#H

2OHFe(OH)

3#H`

hydrolysis of iron (III) (2)

VO2`#2H2OHVO(OH)

2#2H`

hydrolysis of vanadium (IV) (3)

Neglecting polynuclear species, the ionic species of iron(III) present in aqueous solution can change, accordingto pH, from Fe3` to Fe(OH)`` and Fe(OH)`

2, and lastly

precipitate as Fe(OH)3. It is well-known that, if the pH is

less than 1, iron (III) is not yet hydrolysed while, at pH 3,precipitation is complete (Cotton and Wilkinson, 1972;Lide, 1990, 1991; Sillen and Martell, 1964). Therefore ifthe pH of the extract is(1, roughly 3 moles of acidity permole of iron will be produced in titration up to pH 5.6 todetermine total acidity, but if pH is 2 the iron is probablyalready hydrolysed into the "rst of the intermediate

P. Primerano et al. / Atmospheric Environment 33 (1999) 3551}3558 3553

Table 2pH and vanadium, iron and TA content of solutions obtained by multi(*)/single(3) stage aqueous leaching of samples SFM-85, TI-81 andSFM-83

Sample

l/s! (l kg~1) SFM-85 (mol kg~1) TI-81 (mol kg~1) SFM-83 (mol kg~1)(H) (3) (H) (3) (H) (3) (H) (3)

Vanadium (IV) 1 5 0.262 0.614 0.312 0.491 0.100 0.1741 0.179 0.132 0.0961 0.084 0.049 0.016

Iron (III) 1 5 0.036 0.080 0.445 0.878 0.416 0.9201 0.023 0.242 0.3971 0.010 0.142 0.076

TA 1 5 0.804 1.882 1.745 3.133 1.240 2.6201 0.546 0.847 1.1901 0.258 0.441 0.220

pH 1 5 1.86 2.16 1.85 2.25 2.20 2.381 2.01 2.10 2.281 2.21 2.23 2.39

!Liquid/solid ratio.

Table 3pH and vanadium, iron and TA content of solutions obtained by multi(H)/single(3) stage aqueous leaching of samples PG-94 and TI-94

Sample

l/s! (l kg~1) PG-94 (mol kg~1) TI-94 (mol kgv1)(H) (3) (H) (3) (H) (3)

Vanadium (IV) 1 5 0.376 0.690 0.112 0.3271 0.200 0.0831 0.080 0.079

Ferro (III) 1 5 0.220 0.367 0.073 0.1941 0.098 0.0501 0.040 0.036

TA 1 5 1.870 3.200 0.969 2.8751 0.960 0.6931 0.390 0.647

pH 1 5 0.30 0.83 0.25 0.501 0.50 0.331 0.94 0.49

!Liquid/solid ratio.

hydrolytic forms, so the acidity content will be between2 and 3.

Vanadium (IV) is present in the extracts as a vanadylcation (represented to the left of reaction 3) and remainsso up to about pH 3 when it begins to precipitate ashydroxide (Sillen and Martell, 1964). Thus its contribu-

tion to hydrolytic acidity is around 2. In short, the acidityproduced by the hydrolysis of the metallic cations canvary from a minimum of 2 [contribution of the vanadium(IV) and the already partly hydrolysed iron] to a max-imum of 3 (contribution of the Fe3` ion). In calculatingtotal acidity (TA), we must add to this hydrolytic acidity


Table 4HA, SA and pH of solutions obtained by multi(H)/single(3) stageaqueous leaching of ash samples

Sample l/s!(l kg~1)

HA"

(mol l~1)SA#

(mol l~1)pH$ pH%&

PG-94 1(H) 1.412 0.458 0.33 0.30 (0.14)1bis(H) 0.694 0.266 0.57 0.501ter(H) 0.280 0.110 0.96 0.945(3) 0.496 0.144 0.85 0.83

TI-94 1(H) 0.443 0.526 0.28 0.25 (0.10)1bis(H) 0.322 0.371 0.43 0.33 (0.20)1ter(H) 0.266 0.381 0.42 0.495(3) 0.247 0.328 0.48 0.50

!Liquid/solid ratio;"HA "2 V #3 Fe;#SA " TA!HA;$Calculated;%Experimental;&The pH values (0.5 were redetermined by a calibration of

the glass electrode more accurate as before (older values arereported in brackets).

Fig. 1. TA versus vanadium#iron in the aqueous extracts ofsamples TI-81, SFM-83 and SFM-85.

(HA) the free (or sulphuric) acidity deriving from thebisulphate and possibly from the sulphuric acid (SA):

1. TA"SA#HA"[HSO~4]#2[H

2SO

4]

#2 [VO2`]#2[Fe(OH)2`]#3[Fe3`];

2. SA"[HSO~4]#2[H

2SO

4];

3. HA"2[VO2`]#2[Fe(OH)2`]#3[Fe3`].

Unlike TA, which was easily determined in individualextracts, it was not possible to experimentally di!erenti-ate SA and HA as they considerably overlap in neutral-ization. Nevertheless, examination of the pH values mea-sured in the extracts of each sample suggests that SAseriously contributes to TA only in the more recentPG-94 and TI-94 samples, where the pH values recorded(0.25}1) cannot be explained by the hydrolysis ofvanadium and iron in that it should not have started ineither of these. This makes it possible to simplify (3) (inthat the term [Fe(OH)2`] should be close to zero) and tocalculate HA using the concentrations of vanadium andiron determined in the extracts. Its values are set out inTable 4 along with SA values calculated from TA minusHA and pH values calculated from SA. The latter arevery close to experimental pHs albeit with some admiss-ible approximations considering the reduced accuracy ofthe glass electrode in the strongly acidic "eld. These"ndings are not, in our opinion, coincidental and repres-ent quanti"cations of free acidity or what we earlierreferred to as sulphuric acidity (SA), i.e.

SA"TA!2V!3Fe"[HSO~4]#2[H

2SO

4].

This, in other words, means that, from these two mostacid samples, water extracted vanadyl (IV) and iron (III)monohydrogensulphate rather than (or as well as) neu-tral sulphate.

Moving on now to the analysis of the data obtained forthe three less acid samples and attempting to set out totalacidity values determined for each aqueous extractagainst total molar concentrations of V#Fe, we arriveat a linear relationship passing through the axes originwith a slope of 2.4$0.2 (Fig. 1), data which respectivelycon"rm the absence of sulphuric acidity and explain totalacidity as being exclusively due to hydrolytic acidity. Inother words, this means that water has extracted onlyneutral sulphates of V, Fe, Ni, Mg, etc. from the samples,the "rst two of which hydrolyse in an acid "eld.

3.3. General model on the formation of strong primaryacidity from fuels

It is now interesting (and possible) to trace and explainchemical events as they probably occur in combustionand the immediately subsequent cooling phases of thefumes. Let us suppose that coal and fuel oil with sulphurcontent of between 1 and 3% are burned. From theburning of sulphur mainly SO

2is produced but a minim-

al proportion (a few percentage units) of this can beoxidized to SO

3, this oxidation being favoured by the

excess of air and catalysed by in-#ame metal oxides andtheir scales, which stick to the boiler tubes (EPRI, 1983;Goldstein and Siegmund, 1976; Homolya and Fortune,1978; Hu!man et al., 1991a, b; Taghiei et al., 1991). As thetemperature decreases, the extraordinary reactivity ofSO

3as an acid oxide comes into play and combines with

water (to form sulphuric acid) and with the few basicoxides available, i.e. with the iron, vanadium and nickeloxides present in the ashes (to form salts). Thus the


1This must be due to ENEL's known decision to use fuel oilswith low asphaltene content to avoid the use of high-perfor-mance electro"lters that would otherwise be required by themore restrictive limits on particulate emission now in force.

following reactions are probable:

SO3(g)#H

2O(g)PH

2SO

4(l)

hydration of SO3

(4)

(Na2, Mg, Ca)O(s)#SO

3(g)P(Na

2, Mg, Ca)SO

4(s)

neutralization with strong basic oxides (5)

V2O

5(s)#SO

3(g)P(VO

2)2SO

4(s)

NiO(s)#SO3(g)PNiSO

4(s)

Fe2O

3(s)#3SO

3(g)PFe

2(SO

4)3(s)

FeO(s)#SO3(g)PFeSO

4(s)

neutralization with weak basic oxides (6)

In addition, in the ashes of oil-based fuels typicalvanadium reactions can occur which would explain thepresence of vanadium (IV) and sulphur (VI) and theabsence of iron (II) and sulphur (IV). Indeed, once thereaction (6) has taken place, the ashes can: (i) absorb SO

2from the gaseous phase and give rise to a further reaction:

(VO2)2SO

4(s)#SO

2(')P2VOSO

4(s) (7)

reduction from (V) to (IV) of vanadyl sulphate with SO2

(Aurisicchio et al., 1995; Koltho! and Elving, 1961).

and (ii) interact with the ferrous sulphate deriving fromplant corrosion to oxidize it to ferric sulphate:

(VO2)2SO

4(s)#2FeSO

4(s)#2H

2SO

4(l)

P2VOSO4(s)#Fe

2(SO

4)3(s)#2H

2O(l).

reduction with iron (II) (Koltho! and Elving, 1961). (8)

All of these contributions can result in either completeor incomplete neutralization of the sulphuric acid arisingin the ashes. In the case of the burning of coal, thequantity of basic oxides (CaO, MgO, Na

2O, Al

2O

3, etc.,

estimated to be around 10%) is such that they consider-ably exceed quantities of SO

3produced. When oil is used,

strong basic oxides (reaction (5)) are almost totally absentand, moreover, the content (0.01}0.02%) of the weaklybasic oxides making up the ashes (vanadium, iron, andnickel oxides in the main) is of the same order of magni-tude as the aliquot of sulphur oxidized to SO

3(estimated

beforehand to be a few percentage units of total sulphur,in turn assumed to be between 1 and 3%, in the fuel);even when neutralization is quantitatively complete, thesalts produced deriving from a strong acid and a weakbase give rise to acid hydrolysis in aqueous solution.

Therefore, besides sulphur content and excess of air,the factor determining the resulting strong primary acid-ity is the content of basic oxides and their basic strength.This also explains why coal ashes are alkaline and oilashes acid.

Even if the fundamental factor for the formation ofstrong primary acidity still remains the presence of sul-phur in fuel, it is nevertheless interesting to note that it isnot total sulphur content that is the most importantfactor, but combustion conditions } in that these favouror do not favour the formation of SO

3} and the quantity

and basic strength of the basic oxides present to neutral-ize it. Hence the greater dangerousness of oil-based fuelsas sources of strong primary acidity.

Moreover, it must be emphasized that the ash samplescollected more recently (1994) show distinctly higherstrong primary acidity than those representing the pre-vious 10 years despite having comparable sulphur con-tent (Table 1). From this table we can also see that thepossible reason is that the metallic constituents of therecent ash samples amount to between half and three-quarters of previous ones.1 Our results indicate that thisleads to the production of less but much more acidic ashwhich is therefore much more dangerous to the environ-ment.

Finally, if we consider that our oil #y ash samples werecollected from ESP and not from the atmosphere in theimmediate vicinity of the chimney stacks (this kind ofsampling has so far been precluded to us), they maycontain less strong primary acidity than those emittedinto the atmosphere, as aliquots of acids (SO

3, H

2SO

4,

HCl) may penetrate the heat-exchanger and ESP andcondense onto the surfaces of #y ash particles after emit-ted gas cools in the air (Appel et al., 1980, 1982; Brossetand Ferm, 1978; Harrison and Plo, 1983). This will evenincrease the acidity of combustion aerosols really emittedinto the environment and validate further our "ndings.

From examination of the above problem, some obser-vations emerge, which are probably not altogether newbut are certainly little known or little considered in theenvironmental "eld, such as: (i) pH is not su$cient toevaluate environmental acidity; it only represents freeacidity which can be of little consequence compared withother equally fearsome types of acidity. (ii) Fuels with highsulphur content can be extremely dangerous or completelyinnocuous in terms of strong primary acidity. (iii) Thedecisive factor as regards acidity is not so much sulphurcontent as the basic components of the ashes or, to bemore exact, the balance between acid and basic oxides inequivalents. (iv) Whether the basic oxides are strong orweak is also very important. (v) Particulates containingneutral sulphates (or chlorides) of polyvalent cations can,on impact with water, result in dangerous hydrolytic acid-ity. (vi) The sulphur in oil-based fuels is much more dan-gerous than that in solid fuels with high ash content.


4. Conclusions

The results of this study have enabled us to identify thecauses of strong primary acidity formation in combus-tion residues. The root factor is the formation of sulphurtrioxide in signi"cant proportions (for example a fewpercentage units), albeit in much smaller proportionsthan dioxide. The next important factor is the excess use,a very common practice, of oxygen (or air) in relation tocombustion stoichiometry and the contact of hot fumeswith an oxidation catalyst (vanadium, iron and, gener-ally, transition metal oxides). If conditions favouringsigni"cant SO

3formation are present, the next event to

consider is its high reactivity towards basic oxides in thecooling phase. In this respect, three fundamental condi-tions can occur.

Condition A: If a su$cient quantity (or an excess) ofstrongly basic oxides (alkaline or alkaline-earth metaloxides) is present in the fuel ashes, and therefore amongthe available species, SO

3is neutralized and no strong

primary acidity is recorded.Condition B: If only polyvalent or transition weakly

basic metal oxides are available in the fuel ashes but arenonetheless in stoichiometric excess of SO

3, the SO

3is

also neutralized but the salts dragged from the ashes cancause acid hydrolysis on contact either with rain oratmospheric humidity.

Condition C: If these weakly basic oxides should belacking with respect to the complete neutralizationstoichiometry of SO

3(or, obviously, of its hydracid

H2SO

4), both residual sulphuric acidity and hydrolytic

acidity can be found in the ashes.Condition A is typically represented by coal (even with

high sulphur content) because in coal a high excess ofstrong basic oxides is always present in the ashes. Condi-tion B may be represented by Venezuelan or SouthAmerican oils characterized by high sulphur (Hi.S) con-tent, but even more by very high (compared with theaverage oil) vanadium content. Condition C is typical ofcommon Hi.S oils. It should also be pointed out that oilswith high vanadium content can not only catalyze theformation of SO

3, but also activate gaseous SO

2through

the reaction: (VO2)2SO

4(s)#SO

2(g)PVOSO

4(s). It is

appropriate to underline the great signi"cance of thisreaction, since it provides an explanation for thevanadium (IV) sulphate or bisulphate (soluble in water asblue salt) commonly found in oil ashes instead of (or aswell as) vanadium (V) pentoxide, which is little soluble inwater.

It is hoped, by drawing further attention to theseprinciples in the environmental sector, that the need willbe recognized for changes in the laws currently regulatingpollutant emissions, which in some cases are excessivelysuper"cial and summary. We refer, more particularly, tothe fact that current regulations exclusively govern par-ticulate emission into the atmosphere on the basis of

concentration in the fumes despite the need, as this studyshows, for them also to monitor and limit emissions byconsidering acidity and reactivity. It should become com-pulsory to determine the pH and total acidity of partic-ulate emissions, at least in activities which involve thewholesale burning of heavy fractions or of other oilderivatives (diesel oil, naphtha, fuel oil, tar, oil coke). Inthis way all situations of danger to the environment fromstrong primary acidity and, more generally, from aciditycan be identi"ed.

As for remedies, these are implicit in the chemicalcauses of the phenomenon evidenced in this study: itis su$cient to use a strongly basic additive in #ameto limit or eliminate it. It is no mystery, moreover,that the in-#ame insu%ation of a magnesium oxide emul-sion in fuel-oil has already been proposed and has forsome time been in use with good results (EPRI, 1983), butit is no credit to the laws governing the control of envir-onmental pollutants that this remedy was empiricallyidenti"ed and used in power stations to limit plant cor-rosion rather than to limit the emission of acidity into theatmosphere.

Acknowledgements

The authors thank the National Research Council(CNR) for "nancial support and Ente Nazionale EnergiaElettrica (ENEL) for the samples supplied.

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formation of strong primary acidity in combustion particulate: causes and remedies

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