# Steam Gasification of Lignite in a Bench-Scale Fluidized-Bed Gasifier Using Olivine as Bed Material
## 3 Results and Discussion
### Pressure Fluctuation Analysis  
More than 150 acquisitions of pressure fluctuation signals were performed during the six tests discussed above, always depicting the situation exemplified by Figure 7 for test #1. During the preliminary heating of the reactor, under an N\({}_{2}\) flowrate high enough to fluidize the bed, PSDF resulted in dominant frequencies of around 3-4 Hz (Figure 7a), which were compatible with the desired bubbling fluidization regime (usually less than 10 Hz) [24] and were assumed to be characteristic of the fresh olivine bed inventory. As soon as the gasification session started, a series of low-frequency phenomena (<1 Hz) took action with a high power spectral density, partially disguising those related to bed bubbles in the PSDF; the latter were still detectable, maintaining their dominant characteristic frequencies at 3-4 Hz (Figure 7b). The low-frequency phenomena were associated with the peristalic pump feeding water and the instantaneous devolatilization of lignite particles. As further confirmation \begin{table}
\begin{tabular}{c c c} \hline \hline
**Element** & **Concentration (\%)** & **Absolute Error (\%)** \\ \hline Na & 0.705 & 0.037 \\ Mg & 2.399 & 0.016 \\ Al & 0.149 & 0.0046 \\ Si & 3.690 & 0.006 \\ S & 6.577 & 0.004 \\ Cl & 0.113 & \(3\times 10^{-4}\) \\ K & 0.094 & 0.0011 \\ Ca & 12.460 & 0.01 \\ Ti & 0.108 & \(7\times 10^{-4}\) \\ Cr & 0.072 & 2.5e-04 \\ Mn & 0.176 & \(4\times 10^{-4}\) \\ Fe & 5.964 & 0.005 \\ Ni & 0.067 & \(3\times 10^{-4}\) \\ Cu & 0.0143 & \(1.4\times 10^{-4}\) \\ Sr & 0.145 & \(1\times 10^{-4}\) \\ Ba & 0.169 & 0.0011 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: XRF analysis of lignite ash produced during the gasification test. Figure 6: XRD analysis of lignite ash. of this last observation, pressure fluctuation signals were also acquired during post-combustion, when water and lignite were no longer fed. In related PSDF, dominant frequencies clearly reappeared within the range of 3-4 Hz, without any high power spectral density disturbance at less than 1 Hz (Figure 7c). For the case shown in Figure 7, standard deviations of pressure fluctuation signals were 0.98 mbar for preliminary heating (Figure 7a), 3.14 mbar during gasification (Figure 7b), and 0.39 mbar for post-combustion (Figure 7c), with trends and orders of magnitude representative of all tests. The fresh olivine beds during preliminary heating, approaching temperatures of the gasification, had pressure fluctuations with a standard deviation of around 1 mbar. It increased several times during gasification  
Figure 7: Power spectral density function (PSDF) of pressure fluctuations signals from test #1: pre-heating under N\({}_{2}\), T = 767 \({}^{\circ}\)C (**a**): gasification (**b**) with magnification of a 2–6 Hz range (**c**); and post-combustion (**d**) with magnification of a 0–6 Hz range (**e**). (because of the powerful low-frequency phenomena mentioned above), and then returned to the order of 1 mbar in the post-combustion phase. All these observations allowed us to conclude that, for the investigated process conditions, the olivine bed inventory did not undergo modifications able to modify its fluidization quality, as sintering between olivine particles or due to lignite and ashes. This would have caused an increase of the average particle diameter and then a drop in the fluidization quality, detectable by PSDF modifications. SEM analyses confirmed the absence of particle sintering or agglomeration. Figure 8 shows an SEM image of olivine after the test, in which it is possible to see that the dimensions of the particles are approximately between 200 and 400 \(\upmu\)m, similar to the particle size of olivine before the test. No sign of agglomeration or particle sintering is observed from the SEM images. In the tests carried out in this work, the absence of agglomeration issues was probably proof of the suitability of the operating temperatures chosen, which were lower than the ash melting point.