Lignite Ash

The ashes produced during the tests were collected and analysed with SEM/EDS. Figures 4 and 5 show that in some spots of the analysed areas, Si and Mg are found together, probably corresponding to olivine particles still present in the ashes after their separation from the bed material. Furthermore, it was noticed that Ca, which is largely present in lignite ashes [37,38], was often found together with S in some spots analysed, as highlighted by the colored maps showing the presence of single elements in the particles.

The ashes generated during the tests were also analysed with XRF and XRD. The elements detected and quantified with the XRF analysis are reported in Table 4. The value of loss of ignition was 43.28%. The result of the XRD analysis is reported in Figure 6.

The results of the XRF analysis show that the elements present in major quantities are Ca, S, Fe, Si, Mg, and Na, as observed in the results obtained from the SEM/EDS analysis.

Figure 6 shows the di ffraction spectrum of the ashes collected in the reactor after gasification and post-combustion. The broad halo visible at low Bragg angles indicates the presence of amorphous phases typical of ashes [39]. The peaks identified show the presence of K and Ca oxides present in the ashes, and of Si, Mg, and Fe compounds, present in both the ashes and the olivine particles [40]. Furthermore, the phase CaSO4 was identified, as proof of the a ffinity between Ca and S already observed in the results of the SEM/EDS analysis. The observation of Ca and S together, in the characterization analysis of the ashes, could be proof of the capacity of Ca to react with S, as reported in the literature [41,42], and thus to retain the sulphur compounds in the ashes.

**Figure 4.** SEM/EDS of ash from lignite (image 1).

**Figure 5.** SEM/EDS of ash from lignite (image 2).


**Table 4.** XRF analysis of lignite ash produced during the gasification test.

#### *3.4. 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 N2 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 peristaltic pump feeding water and the instantaneous devolatilization of lignite particles. As further confirmation 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).

**Figure 7.** Power spectral density function (PSDF) of pressure fluctuations signals from test #1: pre-heating under N2, T = 767 ◦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**).

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 (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 μm, 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. Furthermore, fluidized-bed technology has the well-known advantage of guaranteeing a good mixing of the materials and thus a uniform distribution of temperatures across the reactor volume, avoiding the presence of hot spots that could be responsible for ash melting.

The evaluation of the agglomerate formation for longer operational times, and therefore with an increased ash content due to accumulation, was not taken into account, because the tests carried out in this work aimed to reproduce the operation of the HTW gasifier, in which the ash produced during the process is discharged from the bottom of the reactor [43], and consequently, the accumulation of high contents of ash does not take place.

**Figure 8.** SEM analysis of olivine after the test.
