88 (2007) 1171–1177www.elsevier.com/locate/fuproc

Fuel Processing Technology

Sulfation of corrosive alkali chlorides by ammonium sulfatein a biomass fired CFB boiler

Markus Broström a,⁎, Håkan Kassman b, Anna Helgesson c, Magnus Berg c,Christer Andersson c, Rainer Backman a, Anders Nordin a

a Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Swedenb Vattenfall Power Consultant AB, Box 1046, SE-611 29 Nyköping, Swedenc Vattenfall Research and Development AB, SE-814 26 Älvkarleby, Sweden

Abstract

Biomass and waste derived fuels contain relatively high amounts of alkali and chlorine, but contain very little sulfur. Combustion of such fuelscan result in increased deposit formation and superheater corrosion. These problems can be reduced by using a sulfur containing additive, such asammonium sulfate, which reacts with the alkali chlorides and forms less corrosive sulfates. Ammonium sulfate injection together with a so-calledin situ alkali chloride monitor (IACM) is patented and known as “ChlorOut”. IACM measures the concentrations of alkali chlorides (mainly KClin biomass combustion) at superheater temperatures. Tests with and without spraying ammonium sulfate into the flue gases have been performedin a 96MWth/25MWe circulating fluidized bed (CFB) boiler. The boiler was fired mainly with bark and a chlorine containing waste. KClconcentration was reduced from more than 15 ppm to approximately 2 ppm during injection of ammonium sulfate. Corrosion probe measurementsindicated that both deposit formation and material loss due to corrosion were decreased using the additive. Analysis of the deposits showedsignificantly higher concentration of sulfur and almost no chlorine in the case with ammonium sulfate. Results from impactor measurementssupported that KCl was sulfated to potassium sulfate by the additive.© 2007 Elsevier B.V. All rights reserved.

Keywords: Biomass combustion; High temperature corrosion; Sulfation; KCl; ChlorOut; IACM

1. Introduction

High temperature corrosion causes expensive shutdowns andrepairs for the owners of heat and power plants. Fuels with highchlorine content such as municipal solid waste or some biomassderived fuels tend to aggravate corrosion by elevating alkalichloride concentrations in the flue gas. Alkali chlorides (mainlyKCl in biomass combustion) also increase deposit formationdue to condensation and stickiness caused by their low meltingtemperature. However, both superheater corrosion and depositformation can be significantly reduced by co-combustion withfor instance coal or peat, or by using additives. The additivescan either prevent the release of gaseous KCl or react with KCl

⁎ Corresponding author. Tel.: +46 90 786 55 56; fax: +46 90 786 95 91.E-mail address: markus.brostrom@chem.umu.se (M. Broström).

0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2007.06.023

to form less corrosive components. Several alkali retainingminerals have been suggested in the literature where potassiumand sodium are sequestered in the bottom ash [1]. Sulfurcontaining additives can react with alkali chlorides and formless corrosive alkali sulfates [2,3]. The sulfates do not have thesame sticky and corrosive properties as the chlorides and theharmful chlorine is prevented from reaching the deposits.

ChlorOut [4] is a concept developed and patented byVattenfall AB. An aqueous solution of ammonium sulfate issprayed into the hot turbulent zone prior to the superheatersaiming at sulfation of corrosive gaseous alkali chlorides in theflue gas. The process can be described as the reactions R1–R3below for potassium, which is the most relevant alkali metal inbiomass combustion, but this simplified reactions are valid forsodium as well. Ammonium sulfate decomposes into NH3 andSO3 (R1), which is more reactive than SO2 [2,3,5]. Theformation of SO3 is favored by oxidizing conditions. Alkali

Fig. 1. Schematic drawing of the Munksund boiler.

Fig. 2. The impactor sampling equipment.

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chlorides are converted into less harmful sulfates in the sulfatingreaction (R2). The ammonium sulfate also reduces NO by theSNCR reaction (R3) [2,3].

ðR1ÞðNH4Þ2SO4ðaqÞ→2NH3ðgÞ þ SO3ðgÞ þ H2OðgÞ

ðR2Þ2KCðlgÞ þ SO3ðgÞ þ H2OðgÞ→K2SO4ðsÞ þ 2HCðlgÞ

ðR3Þ4NH3ðgÞ þ 4NOðgÞ þ O2ðgÞ→4N2ðgÞ þ 6H2OðgÞVattenfall has also developed an instrument called “in-situ

alkali chloride monitor” (IACM) [6–8] which continuouslymeasures both concentrations of SO2 and the sum of KCl andNaCl in a flue gas at superheater temperatures. KCl is thepredominant alkali chloride in biomass combustion, andconsequently the result from IACM is expressed as KCl fromhere on. IACM can for instance be used to control the ChlorOutdosage by measuring the concentration of gaseous KCl. IACMcan also be used for on-line control of the level of KCl duringco-combustion [8].

In the present work, injection of ammonium sulfate wastested in a full scale CFB (circulating fluidized bed) boiler. The

Table 1Operational data of the Munksund boiler

Thermal capacity 96 MWthElectrical capacity 25 MWe

Fuel Bark (N80%, energy), reject (b6%) and wood chipsSteam at SH2 outlet 420 °CSteam at Intrex outlet 480 °C, 60 barSteam production 122 t/hStoichiometry 3% O2 (wet flue gases)

effect of the injected additive on flue gas composition, corrosionrate and deposit formation was investigated with promisingresults. Methods used included corrosion probes, low pressureimpactor, IACM and the existing plant instruments for flue gasmeasurements.

2. Experimental

2.1. CFB plant description

The CFB boiler is situated at SCA:s mill in Munksund, Piteå in northern partof Sweden. It is fired mainly with bark, but also with wood chips and up to 6%on energy bases of a chlorine containing reject (mostly plastics) from a nearbycardboard recycling facility. Fig. 1 shows a schematic drawing of the boiler withChlorOut injection, sampling points and IACM monitoring. The sampling pointwas the same for corrosion probes and impactor measurements. The boiler hastwo ordinary superheaters (SH1 and SH2) and one Intrex (the final superheater)in the loop seal. The bed sand is separated from the flue gas in two parallelcyclones dividing the flue gas in two streams, passing parallel ducts until justprior to the second superheater (SH2), where the flue gas is mixed again. Theseparate flows were utilized in the present work by adding ammonium sulfate onone side only, using the other as an unaffected reference. On the reference side,pure water was injected to avoid the risk of analyzing water effects, if any,instead of that of the additive. Thus, the conditions at the sampling locationswere normal fuel mix at one side and normal fuel mix with ChlorOut on theother side. Typical operational data are given in Table 1.

Injection nozzles for ammonium sulfate were installed prior to the cyclonesat a flue gas temperature of approximately 800 °C. The injected ammoniumsulfate was controlled by IACM to a set value for the KCl. Hereby, potentialoverdosing and increased emissions of NH3 or SO2 was avoided.

Initial tests with impactor and flue gas measurements were performed withand without additive injection. Also four-week corrosion probe measurementson both sides were performed with one-sided injection of the additive.

Fig. 3. The corrosion probe.

Table 2Position and composition of corrosion probe rings (wt.%)

Steel Rings Cr Ni Mo Mn Other

15Mo3 1, 4, 7, 10 – – 0.3 0.6 C:0.2, Si:0.213CrMo44 3, 6, 9, 11, 14, 16 0.9 – 0.5 0.7 C:0.1, Si:0.310CrMo910 2, 5, 8, 13, 15 2.2 – 1.0 0.6 C:0.1, Si:0.3X20CrMoV12 1 12, 18 11.2 0.5 1.0 0.5 C:0.2, Si:0.3, V:0.3Esshete 1250 17 15.0 9.5 1.0 6.3 C:0.1, V, Nb, B

Fe is balance.

Table 3Operating conditions and flue gas emissions

Normal operation With ChlorOut

Load (ton steam/h) 88 87Waste fraction (%) 0.4 1.2ChlorOut (l/h) 0 62 a

NO (mg/MJ) 78 44SO2, stack (mg/mn

3) 0 0IACMKCl (ppm wet) 10 2SO2 (ppm wet) 9 24a 62 l/h corresponds to approximately 100 g S/MWh.

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2.2. Operating conditions and stack emissions

Operational and flue gas data were collected during the tests to verifyrepresentative operation conditions and to document effects of ammoniumsulfate injection. The IACM instrument was used to monitor concentrations ofKCl and SO2 prior to SH2.

2.3. Impactor measurements

Measurements with a Berner type [9] low pressure impactor (BLPI, Dekati,Ltd.) were performed to determine the effect of the additive on the flue gas byanalyzing condensing metal compounds. The effect of ammonium sulfate onsize distribution of solid and condensable particles in the flue gas was alsoinvestigated. The impactor was equipped with 13 different stages where particleswere collected and separated with respect to aerodynamic diameter. Fig. 2 showsa sketch of the impactor sampling equipment.

Flue gas was sampled from the hot zone (approximately 715 °C) prior to thesuperheaters. The gas was quenched to 200 °C in the probe with a cooling rate ofapproximately 3000 °C/s. The simple non-diluting probe was expected togenerate a nucleation mode of KCl with its peak at approximately 35 nm [10].This was within the sampling range of the BLPI, which covers aerodynamicdiameters spanning from 30 nm to 12.5 μm. “Non-diluting probe” refers to thesampled gas being extracted and cooled within the probe without any precedingdilution.

A diluter (DI-1000, Dekati, Ltd.) was used after the probe in this setup toattain sufficiently long sampling periods for representative measurementswithout overloading the BLPI. This dilution had no effect on the cooling rate orintra molecular distances during the critical cooling of the flue gas. The dilutinggas (dry, particle free pressurized air) was preheated (200 °C) to avoidcondensation of water. The dilution ratio was 1:9. Finally after dilution, a part ofthe diluted flue gas was sucked through the preheated (150 °C) BLPI using avacuum pump.

The sampling equipment allowed sampling at sub-isokinetic conditionsgiving an excess of particles with sufficient size or inertia to leave the gasstreamlines. This fraction might therefore be overrepresented leading to a

Fig. 4. Operating conditions and flue gas emissions.

somewhat distorted size distribution. At the same time, the configuration of thesampling equipment was associated with losses of coarse particles due tocentrifugal force in the tube bend and adhesion to the tube walls. Furthermore,the construction of the ejector diluter involved losses of large fraction particles[11]. Based on these considerations, only particles smaller than 5 µm wereincluded in the analysis. This size range was also where effects of sulfationreactions were expected to be found.

BLPI measurements were performed in six intervals of 25 min each; threeduring injection of ammonium sulfate and three during injection of water only.The aluminum substrates of the impactor were weighed before and after theexperiments to determine the mass/size distribution of collected particles. Thesamples were stored in a desiccator until the analyzes were carried out. Collectedmaterial was analyzed using SEM/EDS to determine the elemental compositionwithin each stage. The SEM/EDS instrument was a Philips model XL30scanning electron microscope with an energy-dispersive x-ray spectrometer.

2.4. Corrosion probes

A four-week test was performed with two identical corrosion probesoperating on each side of the boiler, but with ammonium sulfate injection onlyon one side. The corrosion probes (Fig. 3) were temperature controlled gradientprobes with 18 different test rings (Table 2). The metal temperatures weremeasured by thermocouples (TC) and regulators controlled the flows of coolingair on the inside of the probe. A temperature profile, ranging from 400 to 555 °Cwas obtained with the lowest temperature closest to the boiler wall.

The test rings were weighed prior to the experiments, and the thicknesseswere measured with a micrometer at 18 points around the mid circumference ofeach ring. After exposure, the corrosion probes were carefully dismounted,keeping loosened layers of deposits and oxides, if any, together with thecorresponding ring. The rings were weighed again and the weight change due todeposit growth and oxide formation was determined. The rings were embeddedin epoxy, cut in half and the cross sections were polished. Metal loss bycorrosion was determined by measuring the distance from the inside of the ring

Fig. 5. Size distributions of the impactor measurements.

Fig. 6. Relative elemental compositions (molar %) of the different stages of the impactor.

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to the interface between metal and oxide with a measuring microscope. Theoxide and deposit characteristics as well as elemental compositions weredetermined by SEM/EDS analysis of the polished cross sections.

Fig. 7. Rings 1–3 of the corrosion probes without (a) and with (b) ammoniumsulfate injection.

3. Results and discussion

3.1. IACM measurements

In Fig. 4, a combination of operational- and IACM datashows the step response from introducing additive injection.The waste fraction varied between 3–4% during this period.KCl concentration was reduced from more than 15 ppm toapproximately 2 ppm according to the IACM instrument.Furthermore, NO emissions were reduced by 50–60%.

3.2. Impactor measurements

The measurements with the BLPI impactor were carried outduring one particular day. The mass size distributions of sixdifferent impactor samplings were analyzed. A clogged diluterin one of the tests resulted in too low values, and these valueswere therefore excluded from further evaluation. The operatingcases were normal fuel mix with and without ammonium sulfateinjection. Table 3 shows average stack emissions, operatingconditions and IACM data during the impactor measurements.The lower KCl concentration at normal operation, compared toFig. 4, is due to a lower waste fraction during these tests.

3.2.1. Size distributionsSize distributions of the evaluated impactor samplings are

shown in Fig. 5. A significant difference in size distributionwithin the fine particles collected in the impactor was foundwhen ammonium sulfate was added to the hot flue gas.

3.2.2. Elemental compositionSEM/EDS analyzes of the impactor samples are presented as

fractions of the collected mass within each stage in Fig. 6. The

term “rest” covers all other elements detected by the EDS (Mg,Fe, P,Mn, Ti, C) but with the exception of Al, which could not beevaluated since the aluminum substrates of the impactor

Fig. 8. Weight change of the corrosion probe rings.

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interfered with the analysis of the sampled aerosols. StagesN5 μm were not considered relevant to analyze.

More gaseous potassium chloride present in the referencecase resulted in a nucleation mode of salt particles found in thetwo last stages of the impactor (b0.10 μm). These particles wereprobably nucleated during quenching of the gas within thesampling equipment. When ammonium sulfate was added,more potassium (and sodium) was found in the typicalaccumulation mode with a size range of 0.1–1 μm. The totalamount of sulfur also increased. The chlorine concentrations in

Fig. 9. SEM micrographs of the oxides and deposits of ring no

the impactor samples decreased when ammonium sulfate wasadded. This almost complete absence of chlorine in the sub-micron particles when adding ammonium sulfate to the flue gasstrongly indicated sulfating reactions in the flue gas.

Potassium sulfates are solid particles at sampling temperatureand are therefore larger than the newly nucleated KCl. A part ofthe alkali chlorides were probably lost due to their particles beingtoo small for the size range of the impactor. Thereby, morepotassium as a total was found in the impactor when alkali waspresent as sulfates (ChlorOut case). The additional sulfuroriginated from sulfate particles formed in the sulfatingreactions. The different size distributions (Fig. 5) were probablycaused by these sulfates. Additional sodium seen in theChlorOut case indicated effective sulfation of NaCl as well asof KCl. The elevated content of calcium in the submicron rangeindicates a small carryover effect due to bouncing on the non-greased impactor substrates used. Non-greased substrates were aprerequisite for the SEM/EDS analysis.

3.3. One-sided injection

The operating conditions on each side of the boiler at thesampling location were similar to the ones in Table 3. The fluegases were, however mixed prior to SH2 making an evaluation

. 4 (15Mo3, 480 °C) in the reference and ChlorOut case.

Fig. 11. Average corrosion rate of the 18 rings with and without ammoniumsulfate injection.

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of stack emission less relevant. These data are consequently notfurther presented.

3.3.1. Deposit growthVisual inspection of the corrosion probes indicated consid-

erable differences in deposit and/or oxide growth. Fig. 7 showsphotographs of the coolest rings (no. 1–3) on the wind side after4 weeks of exposure without (a) and with (b) ammonium sulfateinjection. The reference case showed considerably moredeposits and scaling tendencies. The white areas beneath thescaled parts of the reference probe deposits (a) were almost pureKCl according to SEM/EDS analyzes.

The results from weighing of the rings before and afterexposure (Fig. 8) confirm the deposit- and/or oxidizinginhibiting effect of the additive seen in the photographs. Theweight change, including both deposit formation and weightchange by oxidation of the metal, was decreased by 69% onaverage with additive injection.

3.3.2. SEM analyzes of corrosion probe ringsSEM analyzes of the corrosion probe rings indicated

considerably less oxide and deposit formation with ammoniumsulfate injection. Analyzes of deposits in the reference caseoften showed a structure with layers of different compositionwhereas it was thinner, denser and more homogenous on theside with the additive. Also a positive effect on corrosion rate(oxide thickness) from ammonium sulfate was seen. Typicaldeposits are shown in Fig. 9.

Fig. 10 shows comparing results of SEM/EDS analyzes ofthe deposit layers in Fig. 9. Only the main deposit formingelements, except for oxygen, are included. Considerably morechlorine was found in the deposits from the reference case. Alsothe different layers found in the reference case showed disparityin composition, where the inner layer had an enrichment ofchlorine, probably present as KCl.

3.3.3. CorrosionFig. 11 shows the average corrosion depth of the 18 rings of

the corrosion probe. The injected ammonium sulfate during thefour-week test resulted in significantly decreased averagematerial loss due to corrosion by 67%, ranging between 47

Fig. 10. Composition of main deposits elements as analyzed by SEM/EDS.Oxygen excluded.

and 93% for the different rings. Trends can be seen in the effectof ammonium sulfate injection and temperature as well as in thechoice of alloy. At temperatures below 500 °C (ring 1–9), thecorrosion followed the trend of the temperature profile (Fig. 11).No clear difference between the materials was seen in thisinterval. Among the remaining rings (11–18), the temperaturewas somewhat higher, but the temperature profile was not assteep as to the left in Fig. 11. Here, some material related effectswere seen instead. The high alloy steel (no. 17, E1250) wasmore corrosion resistant than the others, both with and withoutammonium sulfate injection. The high chromium material X20(rings 12 and 18), however, suffered severely from corrosion inthe reference case, whereas it resisted corrosion well in theChlorOut case. The other materials, 13CrMo44, 10CrMo910and even the low alloyed steel 15Mo3, showed results of nosignificant difference other than that of the additive. Theabsolute figures of these results are probably reproducible onlyunder the current experimental conditions, i.e. for the specificexperimental setup and for relatively short times of exposure.However, it provides an indication of what to expect fromammonium sulfate injection in a boiler. Detailed analyzes of thecorrosion attacks are not included in this paper and will bereported elsewhere (work in progress).

4. Conclusions

Tests with ammonium sulfate as a corrosion and depositioninhibiting additive were performed in a full scale CFB boiler.The most important results from the additive injection are listedbelow.

i) The overall results indicated efficient sulfation ofcorrosive alkali chlorides in the flue gas.

ii) IACM measurements showed that the KCl concentrationin the flue gas was more than 15 ppm during normal fuelmix and a waste content of 3–4%. KCl was reduced toapproximately 2 ppm by the additive.

iii) A reduction in deposit growth by approximately 70% wasdocumented.

iv) Significantly higher concentration of sulfur and almost nochlorine were found in deposits.

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v) Corrosion during the four-week test was reduced byapproximately 70%.

The reduction in deposit formation reported (iii) is validunder experimental conditions, i.e. for the experimental setupand for relatively short periods of exposure. However, itprovides a good indication of the actual conditions in the boiler.The analyzes of the deposits by SEM/EDS (iv) showed reducedcontent of chlorine. Alkali chlorides are known to increasefouling problems and the alkali chloride reduction (ii) by theadditive probably accounts for the reduced deposit formation(iii). Also corrosion rates were reduced when injectingammonium sulfate (v), which is most likely also a consequenceof the decreased alkali chloride concentration in flue gases andthe thereby decreased chlorine content in the deposits. Theresults from the impactor measurements indicated that thesulfation takes place as gas phase reaction between KCl, NaCland sulfur oxides. Size redistribution within the fine par-ticle modes also supports the suggested mechanism of alkalisulfation.

Acknowledgement

The research has been financially supported by VattenfallThermal Technology Program and Business unit Nordic Heat.

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