Energy Fuels 2009, 23, 5655–5662 Published on Web 10/30/2009
: DOI:10.1021/ef900471u
Investigation of Ash Sintering during Combustion of Agricultural Residues and the Effect of Additives Britt-Marie Steenari,*,† Anna Lundberg,† Helena Pettersson,† Magda Wilewska-Bien,‡ and David Andersson‡ †
Department of Chemical and Biological Engineering, Environmental Inorganic Chemistry, Chalmers University of Technology, oteborg, Sweden G€ oteborg, Sweden, and ‡Ecoera AB, G€ Received May 18, 2009. Revised Manuscript Received October 5, 2009
In the process of creating sustainable heat and power production systems it is important to find alternative, renewable fuels that are carbon dioxide neutral. Preferably these fuels should be domestic, thus diminishing the need for transportation. One option could be to use existing residues from local agriculture and food production. Development of combustion methods suitable for such residues is presently being pursued by a number of companies. Because many biomass fuels have compositions that makes them inclined to cause ash sintering problems and emissions of acid gases, there may be a need for the use of additives to decrease such problems. The aim of this work was to examine the ash characteristics of some agricultural crops and residues and to find mixtures of fuels and additives that can form the basis for production of fuel pellets with minimal problems. The work is focused on biomass fuel pellets for smallscale grate-fired combustors. Three additives (limestone powder, kaolin, and sodium bicarbonate) were investigated regarding their effects on the ash melting behavior. The results show that calcium carbonate and kaolin both serve as good additives to prevent the formation of slag. The best antislagging effect was achieved when both additives were used. Sodium bicarbonate can be used as a sulfur binding additive, but this cannot be recommended since it increases the slag formation considerably. The conclusion is that combustion of agricultural crops and residues may be hampered by problems such as slag formation and ash fouling. However, through the use of suitable additives, the ash sintering characteristics can be improved significantly. This means that agricultural residues can be competitive fuels on the energy market in the future.
such has to be refined in order to level out the effects of variable fuel quality. Agricultural residues that may be suitable for use in fuel pellets are straw, hemp, defective grains and seeds, sorting residues, and pressing residues from rapeseed oil production. However, the low ash melting point of such fuels is often causing problems, and the problems can often be traced back to the presence of alkali metal, chloride, and phosphate species in the fuel.1-6 In this paper we focus on potassium since it was the dominating alkali metal in the biomass fuels studied. Potassium is essential for the water balance regulation and other processes in living organisms. Therefore, most plants use potassium as an important nutrient, and it is taken up in the form of dissolved salts from the soil water. Normally, potassium ends up in the ash in the form of chloride (KCl), sulfate (K2SO4), or carbonate (K2CO3). All these compounds
Introduction Biofuels of various kinds are now being introduced on the market since there is an urgent need for alternatives to fossil fuels. The biofuels are renewable, carbon dioxide neutral, mainly domestic, and in some cases residues that would have required a disposal option. The amounts of surplus crops can be substantial. One example is that about 20% of the 5.3 million tons of grain produced in Sweden in 2004 was a surplus that did not go to domestic production of food and feed. However, that situation will not be constant and other utilization options may compete for the biomass intended as a fuel. Some examples are the wood and paper industry that will need a certain amount of wood, the food processing industry requiring agricultural products, and the production of ethanol and other fuels for transportation. This makes it likely that production of heat and electricity will have to be carried out using a more diverse mix of solid fuels than earlier and that many of these fuels will be residues with variable quality. This is positive for the farmers as it will create a better economic outcome of their enterprises, but it is a challenge in the combustion since it introduces problems such as ash melting, fouling of heat exchanger surfaces, and corrosion. Methods to minimize such problems have to be developed. In addition, the monitoring and optimization of the combustion process as
(1) Steenari, B.-M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67–76. (2) Skrifvars, B.-J.; Backman, R.; Hupa, M. Fuel Process. Technol. 1998, 56 (1-2), 55–67. (3) Kaufmann, H.; Nussbaumer, Th.; Baxter, L.; Yang, N. Fuel 2000, 79 (2), 141–151. (4) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Proc. of the 5th Conference on Progress in Thermochemical Biomass Conversion, Tyrol, Austria, September 17-22, 2000, pp 564-572 (5) Fernandez Llorente, M. J.; Diaz Arocas, P.; Gutierrez Nebot, L.; Carrasco Garcia, J. E. Fuel 2008, 87 (12), 2651–2658. € (6) Lindstr€ om, E.; Sandstr€ om, M.; Bostr€ om, D.; Ohman, M. Energy Fuels 2007, 21 (2), 710–770.
*To whom correspondence should be addressed. E-mail: bms@ chalmers.se. r 2009 American Chemical Society
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: DOI:10.1021/ef900471u
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have rather low melting temperatures, and some of them are volatile at normal combustion temperatures (700-1000 °C). Potassium chloride is the most problematic, because it is easily vaporized and transported to the heat exchanger surfaces or other colder surfaces where it condenses.7 There are some ways to avoid the problems caused by alkali metal species in biomass fuels: (1) remove alkali chlorides from the fuel by leaching;8,9 (2) use an alkali metal binding additive or mix fuels to get a better ash composition;1,5,10-15 and (3) add sulfur in elemental form or as a sulfur-containing compound to the combustion zone, thus transforming KCl to K2SO4, which is less corrosive.7,16 All of these methods involve some costs, either due to lower heat output than normal from the combustor or due to the need for investments in equipment or materials. If utilization of the biomass ash as mineral nutrient source in the fields is to be applied, the additives will not hinder that, as unpublished results from our group have shown. However, the cadmium content of the ash used in agriculture has to comply with the limit value of the country. Kaolin has been shown to bind potassium in the form of potassium aluminum silicates.1,12-15,17 This transformation gives a significant increase in the melting temperature of the resulting ash. Increases of a couple of hundred degrees have been reported for straw ash.1,18,19 Since potassium is bound in the kaolin in the combustion zone (reactions 1 and 2), less KCl vapor will be present in the flue gas and, consequently, the ash layers on the heat exchanger surfaces become less corrosive. Kaolin is used in several applications, such as a raw material for ceramics and as filler in paper. The main mineral in kaolin is kaolinite, Al2Si2O5(OH)4. This is a clay mineral with a layer structure that is transformed in several steps during heating. The residue is a more or less amorphous phase called metakaolinite. The potassium species are adsorbed on the surface of the meta-kaolinite and react to the crystalline products KAlSiO4 (kalsilite) and KAlSi2O6 (leucite) according to reactions 1-4.1 The melting point of pure kalsilite has been reported to be about 1600 °C5 and that of leucite to be about 1500 °C.10
The reactions involved in the potassium binding by kaolin are Al2 Si2 O5 ðOHÞ4 þ 2KCl f 2KAlSiO4 þ H2 O þ 2HCl ð1Þ Al2 Si2 O5 ðOHÞ4 þ 2KCl þ 2SiO2 f 2KAlSi2 O6 þ H2 O þ 2HCl
ð2Þ
Al2 Si2 O5 ðOHÞ4 þ K2 SO4 f 2KAlSiO4 þ 2H2 O þ SO3 ð3Þ Al2 Si2 O5 ðOHÞ4 þ K2 SO4 þ 2SiO2 f 2KAlSi2 O6 þ 2H2 O þ SO3
ð4Þ
The exact mechanisms are not clarified yet, so reactions 1-4 above can only be seen as overall reactions. Important questions that remain to be resolved are if the structural rearrangement of kaolin upon heating is crucial for the potassium binding and how the incorporation of the potassium ions is taking place. Potassium containing calcium and magnesium phosphates are other abundant components in ashes from agricultural crops and sorting wastes. The more potassium rich types of phosphates have melting temperatures below 700 °C,6 which makes them troublesome in the combustion unit. It has been shown that the ash melting behavior of phosphate-rich fuels can be significantly improved by addition of lime or calcium carbonate.6 Reactions in the ash lead to formation of various calcium potassium phosphates with high Ca/K molar ratios and melting temperatures of 1100 °C and above.6 In addition, calcium carbonate is known for its ability to bind sulfur dioxide from a gas phase at 600-900 °C and can thus decrease the emission of SO2. Sodium bicarbonate (NaHCO3) is another compound that has been suggested as a suitable additive for binding of SO2. The aim of this work was to examine the ash characteristics and ash melting behavior of some agricultural crops and residues and to suggest suitable mixtures of fuels and additives that can form the basis for production of agro fuel pellets with minimal ash problems, in the first place for small scale combustors, such as the Hotab boilers of 250 kW and 1250 kW used in the Bioagro project.20 The biomass samples were provided by Ska˚nefr€ o AB, producer of cereal grains and grass seeds and host of the Bioagro project. These and similar crop fractions will be used for production of heat for the drying facility at Ska˚nefr€ o AB and possibly also for a local district heating system in a nearby village. More information about this project will be continuously provided on the web page.20 Limestone powder and kaolin were chosen for their abilities to give the ash a higher melting temperature. Sodium bicarbonate was included since it has been suggested as a good sulfur dioxide sorbent that could be used to decrease the sulfur dioxide emissions from the rather sulfur-rich biomass fuels.
(7) Davidsson, K. O.; A˚mand, L.-E.; Leckner, B.; Kovacevik, B.; Svane, M.; Hagstr€ om, M.; Pettersson, J. B. C.; Pettersson, J.; Asteman, H.; Svensson, J.-E.; Johansson, L.-G. Energy Fuels 2007, 21 (1), 71–81. (8) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1996, 10 (4), 177–200. (9) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; J€aglid, U. Fuel 2002, 81 (2), 137–142. € (10) Oman, M.; Nordin, A. Energy Fuels 2000, 14 (3), 618–624. € (11) Ohman, M.; Bostr€ om, D.; Nordin, A.; Hedman, H. Energy Fuels 2004, 18 (5), 1370–1376. (12) Tran, K. Q.; Iisa, K.; Hagstr€ om, M.; Steenari, B.-M.; Lindqvist, O.; Pettersson, J. B. C. Fuel 2004, 83 (7-8), 807–812. (13) Tran, K. Q.; Steenari, B.-M.; Iisa, K.; Lindqvist, O. Energy Fuels 2004, 18 (6), 1870–1876. (14) Tran, K. Q.; Iisa, K.; Steenari, B.-M.; Lindqvist, O. Fuel 2005, 84 (2-3), 169–175. (15) Davidsson, K.; Steenari, B.-M.; Eskilsson, D. Energy Fuels 2007, 21 (4), 1959–1966. (16) Skog, E.; Lindqvist, O.; Folkeson, N.; Johansson, L.-G.; Pettersson, J.; Pettersson, C.; Svensson, J.-E.; Ljungstr€ om, E.; Steenari, B.-M. Inverkan av svaveltillsatser pa˚ o€verhettarkorrosion, emissioner och askkvalitet i en bioeldad anl€ aggning (in Swedish); Report to the Swedish Energy Agency, 2006. ˚ (17) Davidsson, K. O.; Amand, L.-E.; Steenari, B.-M.; Elled, A.-L.; Eskilsson, D.; Leckner, B. Chem. Eng. Sci. 2008, 63 (21), 5314–5329. (18) Ivarsson, E.; Nilsson, C. Report 153; Swedish University of Agricultural Sciences, Department of Farm Buildings: 1988. (19) Kristensen, D. Special Report 154; Swedish University of Agricultural Sciences, Department of Farm Buildings: 1988.
Materials and Methods The biomass fuels used in the experimental work were: (1) sorting waste from wheat, (2) sorting waste from barley, (3) oats grains, (4) rapeseed, (5) residue from rapeseed oil pressing, (6) Poa pratensis (L) Kentucky bluegrass (seeds and (20) Web page describing the Bioagro Project: http://bioagro.wordpress.com.
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sorting waste), (7) Festuca rubra (L) red fescue (sorting waste), and (8) Lolium perenne (L) perennial ryegrass (sorting waste). The term sorting waste is used here for the residues, such as hulls, etc., remaining after the seeds have been recovered. The sample materials are examples of biomass residue fractions that are available in the production of grass seeds and cereal grains. The kaolin used was provided by Imerys Minerals Ltd. under the product name Intrafill C. The particle size is fine; 36% of the particles are smaller than 1 μm and 55% are smaller than 2 μm. The limestone powder (0.2-2 mm) came from the limestone quarry at Ignaberga in the south of Sweden, and the sodium bicarbonate (30-700 μm) was a pro analysi chemical. In the experiments a laboratory oven was used. Ash from the sample fuels, prepared in crucibles with lids at 550 °C, was mixed with additives and heated for 1 h at the temperatures 700, 800, 900, and 1000 °C in open crucibles. In addition, reference samples of fuel ash were subjected to the same heating procedure. The slag formation was then estimated visually according to a graded scale: (1) not or very little affected; (2) affected by partial sintering but still retaining a porous, brittle structure; (3) sintered; (4) very hard, sintered structure; (5) melted. This procedure is not a standardized sintering test, but the intention was to get as much information as possible from a very simple and rather quick test that can be used for making pellet “recipes” in the future. The amount of additive used in the basic tests was three times the stoichiometric amount needed for the expected reaction. This choice was based on earlier results obtained with kaolin.15 This resulted in rather large amounts of additive as calculated in % (w/w) of dry fuel. For most sample fuels the limestone addition was 4.5-6%, but for rapeseed it was 10% and for rapeseed pressing residue it was 17%. The kaolin additions were more varying: 4-5.5% for wheat sorting residue, barley sorting residue, and oats grains; 7-10% for rapeseed, red fescue residues, and rye grass residues; and 15-30% for the remaining fuels. Clearly, some of these additions are too large to be used in a realistic pellet production for economical and practical reasons. However, for the comparison between experiments and for the understanding of the chemical processes it was necessary to carry out all the experiments according to the same principle. The expected reaction products during limestone powder addition have a molar Ca/P ratio of 1.5. This corresponds to formation of Ca3(PO4)2, but can also approximately correspond to formation of more complex phosphates, such as Ca10K(PO4)7. For kaolin addition the Al/K molar ratio in the product KAlSiO4 is 1. In addition, a special series of experiments were made with additions 1-1.5 times the stoichiometric amounts and some selected fuels. In the experiments where both kaolin and limestone powder were added, the respective amounts used were 1.5 times the stoichiometric amounts. The addition of sodium bicarbonate was 1.5 times the sulfur molar content of the fuel ash since the expected product is Na2SO4. The heated ashes and ash-additive mixtures were examined by X-ray powder diffractometry using a Siemens D5000 diffractometer with Cu KR characteristic radiation and a solid state detector. Crystalline compounds were identified by comparison to standards in the Joint Committee of Powder
Diffraction Standards version 2006. This method has a detection limit of about 2 wt % of a crystalline compound in a matrix and does not give the composition of components that are amorphous or is present in very small crystallites (nanometer size). Results and Discussion Ash Sintering and Melting. The melting behavior of the ashes varies significantly between fuel types, but at 900 °C most of the fuels tend to form a sintered ash, as shown in Figure 1. The most severe sintering and melting was observed for wheat waste ash, which sintered at 700 °C and was completely melted at 800 °C. On the other hand, a couple of fuels (bluegrass and rapeseed) seem to be rather good, as seen from the ash problem perspective. The results further indicate that there may be significant differences in sintering propensities between different parts of a plant. One example is the ash from bluegrass seeds that was not measurably affected by the heat treatment even at 1000 °C, whereas the ash from sorting waste from the same grass sintered at 900 °C and melted completely at 1000 °C. Rapeseed ash and rapeseed pressing residue ash showed a similar trend. If the sintering level 3 is taken as the “risk” level for an ash, the fuels wheat waste, barley waste, oats grains, and to some extent bluegrass sorting waste can be considered risky to fire without additives. Possible Correlations between Sintering and Contents of Ash and Levels of Critical Elements in the Fuels. A number of indices that have been used to predict risks for ash sintering in earlier work on coal combustion were tested. None of them gave any useful prediction of the ash sintering occurring in the laboratory experiments. Instead, the contents of ash-forming matter and of some elements (Table 1) that were considered as critical in the respective fuels were investigated for possible correlations with the estimated sintering temperature (sintering level 3 as described above) of the ashes. A low content of ash-forming components in the fuel gives less ash in the combustion unit and thus indicates a lower need for ash removal. This could be an advantage provided that the ash has a favorable composition. However, the results obtained in this work clearly show that low ash content does not imply less ash sintering problems. In fact, the fuel with the lowest ash content, that is, the wheat sorting waste, showed the lowest sintering temperature. When plotting the concentrations of the elements that have been suggested as important for ash sintering against the sintering temperatures for the respective fuel ashes no clear correlations were obtained. This shows that the chemistry involved in ash sintering is complicated and cannot be explained by the presence of one or two components. One example is phosphorus, the presence of which in biomass ash has been pointed out as increasing the risk for a troublesome ash sintering/melting behavior. The concentrations of phosphorus in most of the fuels studied here are between 3 and 4 g/kg dry fuel, with the exception of the rapeseed pressing residue and the rapeseed, which have concentrations of 11.7 and 6.5 g/kg dry fuel and clearly, the contents of phosphorus cannot explain the sintering behavior of the ashes. In fact, data obtained by Bostr€ om and co-workers recently indicate that the presence of phosphorus in rapeseed ash actually can be favorable for the ash melting behavior. The mechanism suggested was that the affinity of base cations toward phosphorus is greater than toward silica, thus withdrawing these ions from forming silicates with low melting
(21) Joint Committee of Powder Diffraction Standards, JCPDSICCD: PDF-4 release 2006; Philadelphia, USA, 2006.
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Figure 1. Ash sintering and melting behavior. Table 1. Ash Content and Content of Some Elements in the Fuels Investigated wheat waste
barley waste
oats grains
%
16.8
17.2
13.3
9
10.1
17.5
10.2
11.3
11.4
% % % % % % % mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
1.7 0.16 0.07 45.9 6.4 2.3 43.5 15 620 110 4300 1000 29
