# Investigation of Pellet Properties Produced from a Mix of Straw and Paper Sludge
## 3 Experimental Research Results and Discussion
### Effect of Paper Sludge on Ash Melting Temperature  
The main purpose of adding paper sludge to straw pellets was to increase the melting temperature of the ash. This was analyzed on the LECO AF 700 device, which monitors the deformation of ash pyramids at a given temperature. Individual characteristic changes in ash are defined by the following temperatures: DT--deformation temperature, ST--shrinkage (softening) temperature, HT--hemisphere temperature, FT--flow (fluid) temperature.  
The measurement was based on the ISO-540 standard and it was necessary to prepare three pyramids from each ash sample. Three measurements were taken and each lasted about 2 h. The furnace temperature increased at a rate of approximately 8 \({}^{\circ}\)C per minute, from the initially defined 750 \({}^{\circ}\)C to the desired 1500 \({}^{\circ}\)C. Samples before and after the measurement are presented in Figure 4. It is already clear that only two samples of ash, namely S100 and SPS-90/10 (the two samples in the upper left of the plate), were completely melted.  
The characteristic ash temperatures of all samples are presented in Table 5. These results present the average values from three measurements. The table of data reports that the higher content of the paper sludge in the samples increases the melting temperature of the ash. The results are satisfactory even at a 10% share of the sludge. Thus, further increasing the sludge content is not relevant in terms of increasing the melting temperature.  
Figure 4: Comparison of samples before (**left**) and after measurement (**right**).  
Figure 3: Higher and lower calorific values of produced samples.  
The temperatures presented in Table 5 are higher than reported in the research of [PERSON] et al. The difference is probably caused by the shape of the specimens, since the tests in the mentioned research were based on the heating of the cylindrical samples (according to CEN/TS 15370-1 based on the ISO 540) [16]. Another factor, as already stated, is the different types of straw.  
A detailed measurement of the sample S100 is shown in Figure 5. At the temperature 1025 \({}^{\circ}\)C, the deformation point DT was determined. The high content of potassium in straw ash significantly affects the melting temperature [5]. The sample softened at approximately 1081 \({}^{\circ}\)C, with a gradual melting of the ash at 1121 \({}^{\circ}\)C and subsequent flow at a temperature of about 1205 \({}^{\circ}\)C.  
Figure 6, for comparison, shows the measurement of the sample SPS-80/20. Deformation of this ash was determined at a temperature of 1285 \({}^{\circ}\)C, a significant increase over the sample S100. This sample containing 20% sludge did not soften and a temperature higher than 1500 \({}^{\circ}\)C would be required.  
The explanation for the rapid increase in the melting temperatures of all samples is the higher proportion of sludge ash. This statement is demonstrated in Table 6.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Straw** & **Paper Sludge** \\ \hline Ash content (\%) & 5.02 & 42.00 \\ Weight of raw material in 10 g sample (g) & 9.00 & 1.00 \\ Ash content in 10 g of sample (g) & 0.45 & 0.42 \\ \hline Ash content in 10 g of sample (\%) & 51.82 & 48.18 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Ash content in 10 g of SPS-90/10 sample.  
Figure 5: Measurement of ash sample S100.  
Figure 6: Measurement of ash sample SPS-80/20.  
In the SPS-90/10 sample, ash is formed by up to 48% sludge ash, although the paper sludge is only 10% by weight in the produced pellets. From the measured values it is obvious that the paper sludge positively influences the melting temperature of the ash and can be used as an additive in the plant biomass combustion processes. This effect is also confirmed in the work of [PERSON] et al. where the melting temperature of the rye straw ash increased upon addition of the Ca-sludge waste from pulp and paper industry [28].