*3.2. Gasification Results*

The results obtained are summarized in Table 3.

**Table 3.** Results of lignite gasification tests.


The length of tests #5 and #6 was shorter compared to the previous tests; however, the gas composition analysed online during the tests reached a steady state after around 10 min, giving stable values for the entire length of the test; a 60 min period was therefore considered sufficient for an evaluation of the results. The amounts of char and ash produced during gasification were evaluated in relation to the total amount of lignite fed during the experimental run. Consequently, the evaluation of their content was not affected by the length of the test.

From the comparison of the results obtained in the first three tests, it is possible to observe that, in general, at higher temperatures, the product gas has a higher quality in terms of the H2O and C conversions and gas yield. Moreover, the gas composition in the tests carried out at 850 ◦C displays an increase in CO and decrease in CO2, probably caused by the higher extent of steam gasification reactions that takes place for higher temperatures, and the lower extent of the WGS reaction, enhanced at lower temperatures (~600 ◦C). Starting from the syngas compositions obtained in the six experimental runs, the equilibrium contents of CO and CO2 were calculated with the software Aspen Plus, in order to compare the experimental and equilibrium compositions. The equilibrium compositions calculated were approximately equal to the gas compositions obtained in the experimental tests, meaning that the WGS reaction had reached equilibrium. For higher temperatures, the char yield is also lower, which is proof of the higher conversion of carbon and thus lower amount of solid un-reacted residual char. Furthermore, the amount of tar produced at higher temperatures is lower compared to the cases with lower temperatures. In the tests conducted at 750 ◦C, the tar content is around 2.5 g/Nm3, while at 850 ◦C, it reduces to 0.5 g/Nm3. Regarding the tar content in general, it was observed that in tests with lignite, the amount of tar produced is lower compared to in similar tests carried out with biomass in the same experimental reactor with olivine as bed material [27,29,30]. Biomass gasification tests in similar conditions carried out at 800 ◦C produced tar contents ≥3300 mg/Nm3, while lignite gasification at the same temperature (test #2) produced a tar content <950 mg/Nm3. This phenomenon could be related to the lower volatile content of lignite compared to biomass (~50% versus ~70%, respectively [27]). In fact, volatile matter has been reported to make organic feedstocks more susceptible to tar formation [31].

The tests with and without air injections in the freeboard were evaluated in a comparison. As expected, in the tests with air injections, there is a higher content of CO2 in the product gas. Furthermore, in the tests with air injections, the H2O conversion was lower, probably because, being a product of combustion, it increases during the reaction. The H2 content and its production in terms of Nl/min are lower compared to the case without air injection. It is possible that some of the produced H2 was consumed in the combustion reactions with the injected O2. The difference between the H2 produced in test #2 (without air injections) and test #5 (with air injections), and their corresponding difference in H2O content in the syngas are both in the order of 0.1 mol/min. This supports the hypothesis that the missing H2 in the tests with air injections has been combusted and converted into additional H2O, as confirmed by the consistency of the reported values of molar flows. In all of the tests, the H2S content was approximately 400 ppm on a dry N2-free basis. The NH3 content, favored by the presence of steam as a gasification agen<sup>t</sup> [32], was around 1000 ppm, with lower values at 850 ◦C. The higher NH3 content at 800 ◦C, noticed both in the tests with and without air injections, was also observed by Xie at al. in gasification experiments on coal macerals [33], and could be related to the combination of two effects: the increase of NH3 production from the N-containing structures in coal enhanced in steam gasification with a higher temperature [32,34,35], and the thermal decomposition of NH3 occurring for increasing temperatures, as found in the literature [36]. For tests #4, #5, and #6, in which air was injected in the freeboard, the presence of O2 increases the possibility of NH3 combustion and consequently decreases its content in the product gas, showing the same trend as a function of temperature.

As mentioned above, in tests #4, #5, and #6, combustion of part of the gas took place, as expected, as a consequence of the air injections, especially those performed in order to increase the temperature in the freeboard and enhance the reactions of tar decomposition. In spite of the combustion reactions, it was observed that the tar content was not really affected by the air injections. The explanation for this could be that in the bench-scale gasifier, the temperature of the reactor is controlled by the electric furnace, so the temperature increase caused by the combustion did not have a relevant e ffect for the promotion of the tar conversion reactions. In addition, due to the reduced dimensions of the bench-scale reactor, the air injections in the freeboard are close to the exit of the gasifier, and consequently, tars could have had too little residence time to decompose. Moreover, the high content of inert N2 in the air injected, which dilutes the O2, could be the cause of the attenuation of the temperature increase due to the combustion, thus reducing the beneficial e ffect of the air injections.

A global mass balance was carried out, taking into account the mass flows of lignite and steam as inputs for the duration of the test. The outputs were calculated as the sum of the masses of the gases produced, the liquid water condensed downstream of the reactor, the tar contents in the samples (reported as the total gas flow), the ash content separated and collected from the bed material after the tests, and the char and residual carbon in the reactor (including the carbon particles deposited on the surface of the filter candle), which were evaluated from the post-combustion carried out after each test.

#### *3.3. Analysis of Materials after the Test*
