Wood Ash Composition As A Function Of Furnace Temperature
Biomass and Bioenergy Vol. 4, No. 2, pp. 103-116, 1993Printed in Great Britain. All rights reserved0961-9534/93 6.00 0.001993 Pergamon Press LtdWOOD ASH COMPOSITION AS A FUNCTION OFFURNACE TEMPERATUREM AHENDRA 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, inductivelyO coupled plasma emission spectroscopy, and X-ray diffraction. Low temperatureO ashwas prepared at 500 C, and samples were heated in a tube furnace at temperature increments to 1400 C.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 CaCO 3 and K2Ca(CO 3)2 wereidentified, whereas in 1300O C ash CaO and MgO were the main compounds. The implications for ashdeposition in furnaces is discussed.Keywords- Ash, Wood, Furnaces.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 transformations occur at higher temperatures whenthe wood is burned or gasified, which mineralsinitiate ash deposition, and which improvements 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)]. Results on the extent of mineral matter transformations with temperatures to 1400 C in air arediscussed in context of ash deposition in boilersutilizing wood and wood waste as fuel.1. INTRODUCTIONWood 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 investi1-3gated, little information is available onmineral matter behavior in wood.Studies of chemical composition of wood ashin the past have primarily been restricted tothe elemental composition 4-12 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 assumption 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/chemical2. EXPERIMENTAL PROCEDURES2.1. Ash preparationLow temperature wood ash was prepared intwo stages. In the first stage, approximately150 g of woodchips were pyrolyzed in a box103
104M. K. MISRA et al.Table 1, LOW temperature ash content of differentwood speciesWood speciesAspenYellow poplarWhite oakWhite oak barkDouglas-fir barkAsh, dry basis (%)0.430.450.871.641.82furnace by heating to 500 C in a closed container. At the termination of devolatilization,the container lid was removed and the residualchar burned at 350 OC, 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 temperature in a tube furnace (Lindberg, Model 54233V). For temperatures lower than 800 OC, analumina crucible was used to heat the ash. Fortemperatures over 800 OC, the ash volume wasfirst reduced by heating it to 800 OC in thealumina crucible and then heating to the required 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 analysis. 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 potassium carbonate which dissociates at a slow rate.Ash samples, for most cases, were prepared attemperatures of 600, 800, 1000, 1200, and1400 OC.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 1300 OC for longenough time, showed signs of melting and resolidification when cooled.2.2. ThermaI analysisThe weight loss of the low temperature ash(LTA) as a function of temperature by thermogravimetric 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 heating rate was about 45 C min -1 in the first stage(T 600OC) and about 25 OC min -1 in the finalstage of the heat-up. Noise levels were significantly reduced by using two low-stiffnesssprings to suspend the platinum crucible fromtube furnace, which contained air at atmospheric 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 sampleTable 2, Parameter values used for the DTAFig. 1. Schematic of experimental set-up for thermogravimetry,
Wood ash composition105Fig. 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 analysesElemental 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 drying 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 usingdistilled, deionized water so that the concentration 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 detectable signals at all angles and that the background noise is kept to a minimum. The sampleswere analyzed in a Scintag/USA X-Ray Diffractometer using a copper target to generate theX-rays (wavelength 0.154 nm).3. RESULTS OF ASH ANALYSIS3.1. Thermal analysisResults of the thermogravimetric analysis anddifferential thermal analysis are presented in
106Fig. 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 composition107Fig. 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 carbonates 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 endothermic 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 (decomposition) 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 650-900 OC is predominantly due to the decomposition ofCaCO 3, and that beyond 900 OC is due to thedecomposition of K2CO3 and in some cases, dueto the dissociation of calcium and magnesiumsulfate.The mass loss observed beyond 900 OC 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 compared to that of CaCO3 and the thermal loadaccompanying decomposition of potassium carbonate is not significant enough to alter theheat-up profiles. The DTA curve correspondingto pine ash in Fig. 3 shows a melting process at850 OC 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 dissociation of this compound.3.2. Chemical analysisThe 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 600 OC. The
Fig. 6. Variation of normalized concentrations of different elements in aspen ash with temperature.109
Fig. 8. Variation of normalized concentrations of different elements in red oak ash with temperature.110
111n.d.—not determined.major 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 toNH 3, NOX and N2 during the combustion ofwood.14,15 Pine and aspen ash have 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 normalized with respect to calcium, because elementalcalcium is assumed not to volatilize from the
112M. 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 normalized concentrations of potassium, sulfur, boron,and copper initially remain constant and thenecline. A significant decrease in the potassiumconcentration is observed at temperaturesgreater than 900 OC. Decrease in the boronconcentration is observed for temperaturesbeyond 1000 OC. 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 oakashFig. 11. Variation of normalized concentrations of different elements in Douglas-fir bark ash withtemperature.
Wood ash compositionThe 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 carbonate and simultaneous volatilization of potassium oxide formed after dissociation. The latteralso leads to a decrease in potassium concentration in the ash. The decrease in sulfur concentration 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 1300 OC 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 corresponding to calcium carbonate. Pine and aspenash contain relatively higher amounts of potassium 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 corresponding 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 over6 0 0 O C .16 Table 5 also indicates that smallamounts of potassium may be present as(K 2SO 4) as the peaks corresponding to this113compound become distinct at higher temperatures. Low temperature ash produced from thewood bark appears to contain predominantlycalcium carbonate at high temperatures thecontent changes to predominantly calcium oxide.4. DISCUSSION4.1. Effect of potassium carbonateThe 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, particularly 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 decomposition 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 836 O C 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 increasing 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
114M. K. MISRA et al.Fig. 12. X-ray diffraction pattern for aspen ash at temperatures of 600 and 1300 OC. The possiblecompounds present are: 1. CaCO3 2. K2Ca(CO3)2 3. K2Ca2(SO 4)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 increase in heat transfer rate.19 The acceleration ofCaCO 3 dissociation in the presence of alkalicompounds has been attributed to improvedthermal contact due to melting of alkali com18pounds, thereby enhancing the heat transferrate to calcium carbonate.4.2. Relative amounts of volatile elementsUpon dissociation at temperatures beyond9 0 0O C, K2 C 03 forms CO 2 and K 2 O, bothexisting as gases at these temperatures. Thedissociation is hence associated with a simultaneous decrease in the potassium content ofthe ash. The amount of potassium present aspotassium carbonate or any other volatile compound can be estimated from the amounts ofpotassium present in the low temperature ashand in the high temperature ash. In general, theFig. 13. Variation of temperature at maximum temperaturedifference with K/Ca.
Wood ash compositionfraction 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
identified, whereas in 1300OC ash CaO and MgO were the main compounds. The implications for ash deposition in furnaces is discussed. Keywords- Ash, Wood, Furnaces. 1. INTRODUCTION Wood fuel for generating heat and power is of interest because wood is a renewable fuel with low ash and sulfur content. However, as with coal, the problems of ash .
Wood ash Wood ash is a good source of potassium. You can use ash as a fertilizer and liming material on vegetable gar-dens, flower beds, lawns, and most shrubs. It’s particularly useful on acid soils low in potassium. The fertilizer value of wood ash depends on the type
glazes. "A suggested starting point for a stoneware ash glaze test would be 40 parts ash, 40 parts feldspar and 20 parts whiting" (2, p. 167). Of these three ingredients, ash was the basic variant. Both types of wood were collected and reduced to ash by burning in an open pit. "Ashes contain alkaline in
vintage wood hd porcelain tile packaging information description item code lbs/box pcs/box sqft/box boxes/skid sqft/skid 6" x 36" vintage wood dune hd porcelain tile 62-700 48.2181239468 6" x 24" vintage wood dune hd porcelain tile 62-500 55.01141440560 6" x 36" vintage wood ash hd porcelain tile 62-703 48.2181239468 6" x 24" vintage wood ash hd porcelain tile 62-503 55.01141440560
9 The Magnum Wood Pellet Stove / Insert is tested for operation with wood pellets with an ash content of no more than 2%. It is recommended that wood pellets with an ash content of 1% or less be used for efficient operation of this unit. Higher ash conte
3 Key Advantages of Wood-based Fuels 1.1 Wood energy is widely used and renewable 1.2 Sustainable wood production safeguards forest functions 1.3 Wood energy is available locally 1.4 Wood energy provides employment and income 1.5 Wood energy supports domestic economies 1.6 Wood energy is modern and leads to innovation 1.7 Wood energy can make a country independent of energy imports
wood (see Section "Role of wood moisture content on corrosion"). 3. Wood preservatives Wood preservatives are chemicals that are injected into the wood to help the wood resist attack by decay fungi, mold, and/or termites. Waterborne wood preservatives are commonly used when the wood may be in contact with humans or will be painted.
Glazes were rich, as if a reproduction of the natural vegetal (rather than mineral) variety. Ash glaze: glaze composed of a high content of wood ash (the first ash glazes were accidental, discovered when wood ash came into contact with pots during firing). Ball mill: a process used to grind materials in glazes.
Voice banking usually involves recording yourself saying a number of phrases, using a computer program. Depending on which voice banking service you choose, the number of phrases you need to record can range from 220-3000. Depending on the strength of your voice and how tired you become, voice banking can take a different length of time for different people. For some it may take a few hours .
