Biomass and Bioenergy Vol. 4, No. 2, pp. 103-116, 1993 0961-9534/93 $6.00 + 0.00Printed in Great Britain. All rights reserved 1993 Pergamon Press Ltd

WOOD ASH COMPOSITION AS A FUNCTION OFFURNACE TEMPERATURE

MAHENDRA K. MISRA,* KENNETH W. RAGLAND * and ANDREW J. BAKER†

*Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706, U.S.A.†U.S.D.A. Forest Products Laboratory, Madison, WI, U.S.A.

(Received 7 March 1992; accepted 12 August 1992)

Abstract- The elemental and molecular composition of mineral matter in five wood types and two barkswas investigated as a function of temperature using thermal gravimetric analysis, differential thermalanalysis, inductively coupled plasma emission spectroscopy, and X-ray diffraction. Low temperature ashwas prepared at 500OC, and samples were heated in a tube furnace at temperature increments to 1400OC.The dissociation of carbonates and the volatilization of potassium, sulfur, and trace amounts of copperand boron were investigated as a function of temperature. Overall mass loss of the mineral ash rangedfrom 23-48% depending on wood type. The mass of K, S, B, Na, and Cu decreased, whereas Mg, P, Mn,Al, Fe, and Si did not change with temperature relative to Ca which was assumed to be constant. Sinteringof the ash occurred, but fusion of the ash did not occur. hr the 600°C ash CaCO3 and K2Ca(CO3)2 wereidentified, whereas in 1300OC ash CaO and MgO were the main compounds. The implications for ashdeposition in furnaces is discussed.

Keywords- Ash, Wood, Furnaces.

1. INTRODUCTION

Wood fuel for generating heat and power isof interest because wood is a renewable fuelwith low ash and sulfur content. However,as with coal, the problems of ash depositionon heat transfer surfaces in boilers and oninternal surfaces in gasifiers still remain. Eventhough mineral matter transformations duringcoal combustion have been extensively investi-gated, 1-3 little information is available onmineral matter behavior in wood.

Studies of chemical composition of wood ashin the past have primarily been restricted tothe elemental composition4-12 as the focuswas largely on the agricultural use of woodash. These include utilization of wood ash as asource for potash production,4 as a limingagent, a source of nutrients for agriculturalplants, 5-7 and as a tannin neutralizing agent inhigh-tannin sorghums to increase the growthrates of chickens.8 Determination of elementalcomposition in ash of leaves and stems has alsobeen carried out to correlate nutrient uptake byplants with the nutrient content of the soil as ameans to monitor plant growth and effect offertilizer supplementation.9-12 A common as-sumption in most of these analyses has been thatthe minerals present are oxides of differentelements. 13 This assumption may be sufficient toidentify the extent of alkalinity of wood ash, butgives little information on the thermal/chemical

stability of the actual compounds that may bepresent in context with the deposition of ashparticles in combustor and gasifiers.

In view of the problems associated withmineral matter in wood, there exists a need toidentify the forms of the minerals, which trans-formations occur at higher temperatures whenthe wood is burned or gasified, which mineralsinitiate ash deposition, and which improve-ments in combustor design and operation needto be made to alleviate the problems associatedwith ash deposition. In this paper we present theresults on the chemical composition of mineralmatter in wood after heating the ash from fivewood species: pine (Pinus ponderosa Dougl. exLaws), aspen (Populus tremuloides Micx.), whiteoak (Quercus afba L.), red oak (Quercus rubra),and yellow poplar (Liriodendron tulipifera L.),and two bark species [white oak and Douglas-fir(Pseudotsuga menziesii (Marib.) Franco)]. Re-sults on the extent of mineral matter transform-ations with temperatures to 1400°C in air arediscussed in context of ash deposition in boilersutilizing wood and wood waste as fuel.

2. EXPERIMENTAL PROCEDURES

2.1. Ash preparation

Low temperature wood ash was prepared intwo stages. In the first stage, approximately150 g of woodchips were pyrolyzed in a box

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104 M. K. MISRA et al.

Table 1, LOW temperature ash content of differentwood species

Wood species Ash, dry basis (%)

Aspen 0.43Yellow poplar 0.45White oak 0.87White oak bark 1.64Douglas-fir bark 1.82

furnace by heating to 500°C in a closed con-tainer. At the termination of devolatilization,the container lid was removed and the residualchar burned at 350OC, which took 5--8 h tocompletely burn at this low temperature. Theyield of ash was between 0.75 g and 1.5 g,depending on the ash content of the wood. Theamount of ash obtained by this method fromsome of the woods is listed in Table 1 as apercentage of the initial mass of wood.

The low temperature ash (LTA) formed bythe procedure outlined above was used forthermal and chemical analyses. For chemicalanalyses samples were obtained by heating thelow temperature ash to the required tempera-ture in a tube furnace (Lindberg, Model 54233-V). For temperatures lower than 800OC, analumina crucible was used to heat the ash. Fortemperatures over 800OC, the ash volume wasfirst reduced by heating it to 800OC in thealumina crucible and then heating to the re-quired temperature in a platinum crucible. Thiswas done primarily to eliminate any possibilityof alumina reacting with the alkali compoundspresent in the ash at elevated temperatures andtherefore affecting the results of chemical analy-sis. The samples were held at the requiredtemperatures for over an hour to allow adequateheat-up time and to ensure completion of anytransformation that may have been initiated atthe sample holding temperature. This is particu-

larly important for pine and aspen ash thatcontain relatively higher proportions of potass-ium carbonate which dissociates at a slow rate.Ash samples, for most cases, were prepared attemperatures of 600, 800, 1000, 1200, and1400OC.

Most of the ash types did not show any signof melting when heated. At temperatures over800OC, the ash showed signs of sintering but thelump of ash formed would crumble under aslight pressure. The extent of sintering increasedwith the increase in temperature and was relatedto the amount of alkali metals present in theash. Some of the ash types, especially those ofpine, when heated to over 1300OC for longenough time, showed signs of melting and re-solidification when cooled.

2.2. ThermaI analysis

The weight loss of the low temperature ash(LTA) as a function of temperature by thermo-gravimetric analysis (TGA) is shown in Fig. 1.A platinum crucible containing LTA wassuspended in a tube furnace by means of aplatinum wire from an electronic balance (CahnElectrobalance Model 7500). Outputs from thefurnace thermocouple and the balance werestored in a PC by means of a data acquisitionsystem (Metrabyte DAS8). The amount of ashplaced in the crucible was about 40 mg whichwas small enough to minimize the temperaturelag between the sample and the furnace, and yetthe observed change in the mass was significantcompared to the background noise. Furnacecontrol parameters were set such that the heat-ing rate was about 45°C min-1 in the first stage(T < 600OC) and about 25OC min -1 in the finalstage of the heat-up. Noise levels were signifi-cantly reduced by using two low-stiffnesssprings to suspend the platinum crucible from

Fig. 1. Schematic of experimental set-up for thermo-gravimetry,

tube furnace, which contained air at atmos-pheric pressure.

Differential thermal analysis (DTA) of theash samples was carried out in a Perkin-ElmerDTA System 1700 instrument to determinethe presence of exothermic or endothermicreactions. Approximately 20 mg of the sample

Table 2, Parameter values used for the DTA

Wood ash composition 105

Fig. 2(a), TGA results for low temperature ash prepared from different wood species.

30 mg of aluminum oxide (referencematerial) were heated in the instrument furnaceunder controlled conditions and in an inertenvironment provided by argon at a constantpurge rate of 20 cc min-1. Values of differentparameters used in these experiments are listedin Table 2. Heat-up temperatures were restrictedto a maximum of 1300°C as the samples werecontained in small alumina cups and keepingthe temperatures below 1300OC ensured that thesamples did not react with the liners.

2.3. Chemical analyses

Elemental and chemical compositions of thewood ash were obtained using InductivelyCoupled Plasma Emission Spectroscopy(ICPES) and X-Ray Diffraction (XRD).Samples for ICPES were prepared by first dry-ing the ash in an oven and then dissolvingapproximately 100 mg of the dried ash in 4 mlof reagent grade, concentrated hydrochloricacid. The mixture was left standing for a coupleof hours for complete dissolution. This solutionwas later diluted to approximately 100 g using

distilled, deionized water so that the concen-tration of various elements was within the linearrange of detection for the ICPE Spectrometer.The solution was analyzed for concentrations ofP, K, Ca, Mg, S, Zn, Mn, B, Al, Fe, Si, and Naat the Soil & Plant Analysis Laboratory inMadison, WI.

Samples for XRD were first finely ground andthen mounted on a glass slide using an adhesivelayer to hold the ash on to the glass surface. Thepowder was ground fine to ensure randomorientation of the crystals so that there aresufficient amount of crystals to generate de-tectable signals at all angles and that the back-ground noise is kept to a minimum. The sampleswere analyzed in a Scintag/USA X-Ray Diffrac-tometer using a copper target to generate theX-rays (wavelength = 0.154 nm).

3. RESULTS OF ASH ANALYSIS

3.1. Thermal analysis

Results of the thermogravimetric analysis anddifferential thermal analysis are presented in

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Fig. 2(b). TGA results for low temperature ash prepared from different wood species.

Figs 2 and 3. TGA results shown in Fig. 2 wereobtained after filtering out the high frequencycomponents (noise) and smoothing the raw datausing the Fourier transform method. Theseresults, in general, show an initial small massloss at low temperatures and a more significantmass loss at temperatures over 650°C. For pine,aspen, and white oak, the mass loss at highertemperatures appears to be taking place in twoor more steps. The observed mass loss fordifferent ash types at different stages are listedin Table 3.

The initial mass loss observed at temperatureslower than 200°C is due to the evaporation of

● Mass loss was insignificant compared to backgroundnoise.

Wood ash composition 107

Fig. 4(a). Variation of calcium concentrated with themerature for different ash types.

water adsorbed by the ash when stored for aperiod of time after preparation. The mass lossobserved at tempertitures over 600OC has beenfound to be due to the decomposition of carbon-ates of both calcium and potassium. This wasconfirmed by the DTA results shown in Fig. 3.The heating cycle in the DTA results for all ashshow a downward deviation of the temperaturedifference line indicating occurrence of an endo-thermic process. This process is irreversible as itdoes not show up in the cooling cycle carriedout immediately after the heating cycle. Theonset and duration of the processes in Fig. 3coincide well with the TGA observations shownin Fig. 2. It is interesting to note that thetemperature range over which the reaction (de-composition) occurs, as seen from both TGAand DTA results, is similar for all ash types, Itwill be shown later lhat lhe mass loss observedin the temperature range of 650-900OC is pre-dominantly due to the decomposition ofCaCO3, and that beyond 900OC is due to thedecomposition of K2CO3 and in some cases, dueto the dissociation of calcium and magnesiumsulfate.

The mass loss observed beyond 900OC inTGA results for pine and aspen ash do not showup as endothermic peaks in the DTA resultsfor these ash types. This is probably because ofthe slower rates of dissociation of K2CO3 com-pared to that of CaCO3 and the thermal loadaccompanying decomposition of potassium car-bonate is not significant enough to alter theheat-up profiles. The DTA curve correspondingto pine ash in Fig. 3 shows a melting process at850OC as the endothermic peak associated withthis process appears during both the heating andcooling cycles. This process appears to be due tothe melting of K2CO3, since cooling is startedimmediately following the heating cycle, therebynot allowing sufficient time for complete dis-sociation of this compound.

3.2. Chemical analysis

The concentration of different elements andthe variation with temperature for different ashtypes was determined by ICPES, and the resultsare presented in Figs 4 to 11. Table 4 lists themeasured concentrations of various elements inlow temperature ash heated to 600OC. The

Fig. 6. Variation of normalized concentrations of different elements in aspen ash with temperature.

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Fig. 8. Variation of normalized concentrations of different elements in red oak ash with temperature.

110

major

n.d.—not determined.

elements in the wood ash are calcium,potassium and magnesium. Sulfur, phosphorusand manganese are present at around 1%. Iron,aluminum, copper, zinc, sodium, silicon, andboron are present in relatively smaller amounts.Oxygen and carbon are also present but are notdetermined by ICPES. The nitrogen content inwood ash is normally insignificant due to theconversion of most of the wood nitrogen toNH3, NOX and N2 during the combustion of

wood.14,15 Pine and aspen ash have

111

higheramounts of potassium compared to poplar oroak ash. The other alkali metal, sodium, isgenerally low in all ash types with the exceptionof the poplar which had 2.3% Na.

Figure 4 shows the variation of calcium withtemperature for different species, and Figs 5 to11 show the variation of other elements normal-ized with respect to calcium, because elementalcalcium is assumed not to volatilize from the

112 M. K. MISRA et al.

ash, and any decrease in the concentrations ofother elements will become evident in suchplots. The normalized concentrations of mostelements, with the exception of potassium,boron, and sulfur, remain constant with anincrease in temperature, and hence are retainedin the ash at higher temperatures. The normal-ized concentrations of potassium, sulfur, boron,and copper initially remain constant and then

ecline. A significant decrease in the potassiumconcentration is observed at temperaturesgreater than 900OC. Decrease in the boronconcentration is observed for temperaturesbeyond 1000OC. Sulfur decreases beyond1000–1100 O C in pine, aspen, and white oak ash,but must be heated to higher temperatues tovolatilize sulfur from poplar and red oakash

Fig. 11. Variation of normalized concentrations of different elements in Douglas-fir bark ash withtemperature.

Wood ash composition 113

The increase in calcium concentrations inFig. 4 at temperatures below 900OC is primarilydue to the decomposition of calcium carbonateand at temperatures beyond 900OC the increaseis due to the dissociation of potassium carbon-ate and simultaneous volatilization of potass-ium oxide formed after dissociation. The latteralso leads to a decrease in potassium concen-tration in the ash. The decrease in sulfur concen-tration is thought to be due to the dissociationof calcium sulfate and potassium sulfate. Thereason for the decrease in boron and copperconcentrations was not investigated becauseonly trace quantities were present.

The results of ICPES were used with XRDanalysis to identify the minerals present in woodash. Typical XRD patterns at temperatues of600 and 1300OC are shown in Fig. 12, and alist of the compounds identified in ash fromdifferent woods is given in Table 5. The lowtemperature ash shows strong peaks corre-sponding to calcium carbonate. Pine and aspenash contain relatively higher amounts of potass-ium compared to poplar ash and show strongpeaks corresponding to K2Ca(CO3)2. Pine ashcontains calcium manganese oxide, aspen ashhas sulfates of calcium and potassium, andpoplar ash, silicates of K, Mg, and Ca. Athigher temperatures, with the dissociation ofcarbonates, XRD patterns show predominantpresence of calcium and magnesium oxides. Inaddition, pine ash being richer in manganeseshows the presence of calcium manganese oxideand manganese oxide. Similarly, poplar, beingricher in sodium, displays weak peaks corre-sponding to sodium calcium silicate. It appearsthat when the ash is left standing in air, calciumoxide reacts with atmospheric water vapor toform calcium hydroxide, however calciumhydroxide is unstable at temperatures over600OC.16 Table 5 also indicates that smallamounts of potassium may be present as(K2SO4) as the peaks corresponding to this

compound become distinct at higher tempera-tures. Low temperature ash produced from thewood bark appears to contain predominantlycalcium carbonate at high temperatures thecontent changes to predominantly calcium ox-ide.

4. DISCUSSION

4.1. Effect of potassium carbonate

The effect of potassium on other compoundsis seen in the results obtained in the DTA of ashfrom different wood species. The second ordervariations seen in the mass loss profiles with ashtype (Fig. 2) is reflected in the DTA results(Fig. 3) as a shift in the temperature of theminima of the valleys (maximum temperaturedifference between the sample and reference).Since the sample weight and other experimentalconditions were the same for all ash, the shift inthe temperature of minima was associated withthe difference in chemical composition, particu-larly the relative amounts of potassium andcalcium. The justification for this argument isbased on the observations of Malik et al.17 andHuang and Daugherty 18 that a trace amount ofpotassium carbonate in limestone (K/Ca~0.01and 0.07, respectively) can accelerate the de-composition of calcium carbonate.

The variation in the temperature at maximumtemperature difference with K/Ca, observed inFig. 3, is shown in Fig. 13. It is seen that thetemperature at the maximum temperaturedifference is 836OC for oak which has aK/Ca = 0.165, and this temperature reduces to788OC for pine which has a K/Ca ratio of 0.56.These results indicate that the presence of in-creasing amounts of alkali compound can lowerthe decomposition temperature of CaCO3. Thiswas confirmed in separate experiments wheretemperatures at the minima for pure CaCO3 andfor a CaCO3/K2CO3 mixture were compared.The temperature at the maximum temperature

114 M. K. MISRA et al.

Fig. 12. X-ray diffraction pattern for aspen ash at temperatures of 600 and 1300OC. The possiblecompounds present are: 1. CaCO3 2. K2Ca(CO3)2 3. K2Ca2(SO4)3 4. CaO 5. MgO 6. Ca2SiOd.

difference for CaCO3/K2CO3 mixture (K/Ca =0.95) is also shown in Fig. 13. Although the trendshown by the mixture appears to be differentfrom that shown by ash, and this may be due toother elements in ash, it is clear that a presenceof alkali carbonate accelerates the decompositionof calcium carbonate. Dissociation of calciumcarbonate appears to be enhanced with an in-crease in heat transfer rate.19 The acceleration ofCaCO3 dissociation in the presence of alkalicompounds has been attributed to improvedthermal contact due to melting of alkali com-pounds, 18thereby enhancing the heat transferrate to calcium carbonate.

4.2. Relative amounts of volatile elements

Upon dissociation at temperatures beyond9 0 0OC, K2C 03 forms CO2 and K2O, bothexisting as gases at these temperatures. Thedissociation is hence associated with a simul-taneous decrease in the potassium content of

the ash. The amount of potassium present aspotassium carbonate or any other volatile com-pound can be estimated from the amounts ofpotassium present in the low temperature ashand in the high temperature ash. In general, the

Fig. 13. Variation of temperature at maximum temperaturedifference with K/Ca.

Wood ash composition 115

fraction of any volatile element with respect toits content in low temperature ash, i.e. i/iO, canbe calculated from,

where subscript o refers to concentration in lowtemperature ash, subscript ƒ denotes concen-tration in high temperature ash, and Ca rep-resents concentration of calcium. The derivationof this expression is based on the reasonableassumption that all of calcium is retained in theash at high temperatures, and hence can be usedas a factor to account for mass change as aresult of dissociation of carbonates at highertemperatures. Estimates of volatile potassium asa percentage of total potassium present in differ-ent wood ash are listed in Table 6 along withsodium, sulfur, and boron. Unlike potassium,these latter elements are present in much smallerquantities and their contribution, if any, to theinitiation of ash deposition may be over-shadowed by that of potassium compounds.

In the absence of substantial XRD evidence,the process leading to a reduction in sodiumconcentrations in pine and oak ash is presumedto be similar to that of potassium, namely,dissociation of sodium carbonate and sub-sequent volatilization of sodium oxide. Re-duction in sulfur concentration at hightemperatures is probably due to the dissociationof sulfates of calcium, magnesium and potass-ium, however the form of the sulfur cannot besubstantiated in the low temperature ash ana-lyzed so far.

4.3. Deposition characteristics of ash in com-bustors

Potassium, sodium and sulfur in the wood ashplay a key role in formation of deposits. Someof the calcium, potassium, and magnesium maybe taken up from the soil as sulfates and phos-phates. These minerals are then transported andstored both in organic and inorganic forms. Itis reported that potassium is present primarilyin solution in cell vacuoles.l4 calcium is presenteither in combined form in the cell wall or ascrystalline calcium oxalate in cytoplasm,14.20

magnesium primarily in combined form inorganic molecules of which chlorophyll andproteins are examples,l4 and silicon as depositson cell wall.14.20 The mechanism by which theminerals are released as ash during the combus-tion of wood is not clear, but it is reasonable toassume that the conversion depends upon the

combustion temperature and the immediate en-vironment. Carbonates are presumably formedat low temperatures in a quiescent atmospherewhen the combustion products, primarily car-bon dioxide, surround the wood grains. Highyields of carbonates indicate that these con-ditions are prevalent during the low temperatureashing procedure followed in this study. Ashformed at high temperatures in an oxidizingatmosphere consist primarily of metal oxides.Ash composition can also be modified by thepresence of silicon, manganese, iron, or alumi-num which are capable of forming acidic oxideswhen the basic oxides are present in associationwith these oxides to give ceramic-like com-pounds in the ash. This is evident in the resultsobtained for both aspen and pine when thepresence of silicon in the former yields silicatesand the presence of manganese in the latteryields manganates.

Ash deposition in combustors and gasifiersmay be initiated by alkali compounds thatgenerally have low melting points. At tempera-tures starting at about 900OC, deposition can beinitiated by the molten potassium carbonate andsulfate which adhere to the colder metallicsurfaces and provide a sticky layer to trap othersolid particulates such as oxides of calcium andmagnesium. Other XRD tests of aspen ashdeposits which we have done have clearly ident-ified K2SO4, CaO and MgO as the predominantcompounds in the deposits. The undissociatedpotassium carbonate may dissociate on themetallic surface after prolonged exposure tohigh temperatures, and the potassium oxideformed may react with the surface elementsto form a chemical bond that holds the depositsto the surface. At higher temperatures, dis-sociation of potassium carbonate and sub-sequent formation of potassium oxide vaporwill be accelerated. On lower temperatureheat transfer surfaces KOH21 and K2CO3 areformed.

With regard to furnace design, the wood ashtest results suggest that in order to minimizedeposit formation, the furnace temperatureshould be held below 900OC wherever possibleso that the volatilization of potassium andsulfur compounds will be restricted.

5. CONCLUSIONS

When low temperature wood ash was heatedto 1300OC, a mass loss of 22.9-47.8% wasobserved depending on the wood type. For

116 M. K. MISRA et al.

600OC ash CaCO3 and K2Ca(CO3)2 were ident-ified, whereas in the 1300°C ash CaO andMgO were the main compounds. Bark ash at600OC is primarily CaCO3 at 600OC and CaO at1300OC. Carbonates of calcium and potassiumdissociated at 700-900OC depending on thewood type. Potassium volatilization began at800-900 OC, and sulfur volatilization began at1000-1200 OC. Copper and boron began volatil-ization at about 1000OC. When heated to1300OC, potassium decreased by 63%. to 90%and sulfur by 7% to 55% depending on thewood type. Potassium and sulfur compounds inwood ash play a key role in the formation ofdeposits.

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