Proceedings World Bioenergy 2010

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It is seen that the total data set, existing of 164 products, have a broad range of differences in ash content and composition. The lowest ash values are found for soft wood consumer pellets (0,20-0,64%). Hard, mixed and waste wood pellets have ash contents between 0,8-2,5%. Miscellaneous non-pellets woods (such as chips, logs, saw dust, flour, bark) have been studied with ash contents in the range of 0,23-5,9%. Non-wood plant based products were studied with ash contents varying between 0,8-14,7%. Waste stream or sludge products had ash contents of 3,5-46%. The main elements of wood based products are calcium, silicon, potassium and magnesium. Calcium contents range from 386 mg/kg for a soft wood pellet to 12990 mg/kg for a product consisting of mixed bark. The waste stream and sludge products are much more divers in composition then the wood or plant based products, having also significant differences in Al, Fe, Na and P contents. Typically sewage sludge products have high iron contents in the range of 10-20 g/kg. The sintering melting temperatures (SST) are found in the range of 578°C for rye straw to 1585°C for kenaf. One has to take into account, while interpreting the results, that the determination of ash melting temperatures can have a standard deviation of up to 50°C. As expected, the melting T of ash of soft wood consumer pellets are higher than of the less clean industry pellets. Typically low melting ash is obtained from grasses and cereal based products (non wood based plants). Figure 2: %Ca of total ash versus ash melting temperature of all products. Logarithmic fit is shown without ash consisting of mixed ash from waste and sewage sludge ash [10]. In figure 2 the ash melting T is shown as a function of %Ca of the total ash for the complete data set. It is known from literature that calcium plays an important role in the ash melting behavior. Authors [9,11,18,19] describe the increase of ash melting T by increasing the Ca content of incineration ash. However, it is clear that calcium content itself is not enough to predict ash melting temperatures of the products. In particular waste, sludge and non-wood plant based products show less correlation between T and %Ca of the ash. The ash melting temperature is correlated to all of wt% per element in the incineration ash (Ca, Mg, Fe, Si, Al, P, Na) for the complete data set studied (n=164). Clearly seen is that calcium is the most important element for affecting ash melting leading to an increase in ash melting T. Silicon and potassium are the 2nd and 3rd most important elements affecting ash melting by lowering the melting T. Fe and Al seem to have a limited effect while, within the data set studied. Na, P and Mg 90 world bioenergy <strong>2010</strong> show hardly any significant effect. The results were confirmed with principle component analyses (PCA). Remarkable is that magnesium shows on average for this data set almost no negative influence on ash melting. In literature some authors have stressed the importance of magnesium in lowering ash melting T but others also have seen increased melting T. Of course, one has to take into account that probably also the interactions between the various elements play an important role. Mixed oxides will all have their different melting characteristics depending also on thermal-physical history of the ash. It is very likely that the total composition of the ash is of great importance for the ash melting behavior. Therefore, it is thought that the ratios of various elements are important. In figure 3 the most important example is given of ash melting T as function of a typical ratio of elements (Ca/[K+Si]). Figure 3: Ratio combined elements (mg/kg product) versus ash melting T of all products. It is seen that again, as expected, in particular calcium is having a positive influence on the effect of Mg, Si en K on the ash melting. The Ln-fit of the ratios Ca/K, Ca/Si en Ca/Mg show a reasonable correlation. The effect of calcium on sodium is less dominant. It is thought that calcium containing silicates have higher melting points than mixed Mg-K-Na silicates. The presence of Fe en Al probably gives a slight increase in the melting of mixed silicates compared to silicates, which have high Na and K content. Mg and P show no correlation with Si. Therefore, it is seen that the Ln of the ratio of the most dominant elements Ca/(K+Si) gives a good correlation with a R 2 of 0,65. The correlation with ratios without Si, K or Ca are worse or even bad. A slight improvement is found for ratios were Al or Fe are included as a positive effect and Na as a negative effect on the Tmelt. However, the improvements are small. Including Mg gives no significant improvement in the fit compared to the ratio Ca/(K+Si). The prediction using the Ln fit of the ratio Ca/(K+Si) is already good as well. Research was extended by using statistics (linear regression analyses) to gain more insight in the interactions between the elements. The main results are shown in figure 4 and 5.
Figure 4: Statistical analyses (linear regression) results. One could say that Ca, K, Si are indeed the most important elements (of the studied elements in this data set) for determining the ash melting T. Apparently, Mg plays less a role. In particular, the prediction of the ash melting T of the wood pellets is very good. Most outliers are from some non-wood plant based products such as kenaf and hemp or waste products. Figure 5: 3-D graphical representation of the model Ln Tmelt = 7,24 -0,33*Si-0,70*K+1,28*K*Ca. high T = right below (high Ca), low T = top left (high K). An extensive comparison of various literature fits (see figure 6 - more details of this study can be found in reference [20]), has been made, that have been taken from literature references [9,10,19]. It is obvious that the fits are reasonable for the limited data set used in the papers [9,10,19]. However, the fits do not give useful predictions for our more broad and larger data set. But they do not give a good, more universal applicable, prediction of the ash melting temperature. Some formulas make use of polynomial fits. This leads to large deviations of the predictions for products with large differences in ash compositions. The data set in the fit used to design the formulas was probably too small. Some formulas are already better but still there are large deviations and multiple outliers. According to most formula calcium and potassium are important as in agreement with our findings. The role of magnesium is also taken into some fits, while our findings show that magnesium plays not a dominant role. More important is the situation if silicon is left out of the equation. One formula takes into account silicon and calcium, but they do not take into account the effect of potassium. This is probably due to the fact that K is not a very dominant component in the incineration ash the authors studied. However, in lignocellulosics, such as wood pellets and plant based bio-energy products, this can be an important component of the ash. Probably a linear fit is also not the best option looking at the physical phenomenon it needs to describe. Figure 6: Comparison of formulas from literature and the new formula based on the Ca/(K+Si) ratio. Top: formula based on synthetic ash. Middle: fit based on lignocellulosics. Bottom: formulas based on waste-sludge ash. Figure 7: Influence addition FlourBond on pellet processing and properties. Pressing aids are used to obtain improved pellet processing and quality. A pressing aid can have effect on the ash melting. The effect of FlourBond , a pressing aid high in Ca, on soft wood pellets was studied, starting by comparing two properties, i.e. abrasion (%fines) and world bioenergy <strong>2010</strong> 91
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MaIN spONsOR: WORLD BIOENERGY 2010
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ORGaNIsED BY: The swedish Bioenergy
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