# Steam Gasification of Lignite in a Bench-Scale Fluidized-Bed Gasifier Using Olivine as Bed Material
## 3 Results and Discussion
### Gasification Results
Biomass gasification tests in similar conditions carried out at 800 \({}^{\circ}\)C produced tar contents \(\geq\)3300 mg/Nm\({}^{3}\), while lignite gasification at the same temperature (test #2) produced a tar content \(<\)950 mg/Nm\({}^{3}\). This phenomenon could be related to the lower volatile content of lignite compared to biomass (\(\sim\)50% versus \(\sim\)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 CO\({}_{2}\) in the product gas. Furthermore, in the tests with air injections, the H\({}_{2}\)O conversion was lower, probably because, being a product of combustion, it increases during the reaction. The H\({}_{2}\) content and its production in terms of NI/min are lower compared to the case without air injection. It is possible that some of the produced H\({}_{2}\) was consumed in the combustion reactions with the injected O\({}_{2}\). The difference between the H\({}_{2}\) produced in test #2 (without air injections) and test #5 (with air injections), and their corresponding difference in H\({}_{2}\)O content in the syngas are both in the order of 0.1 mol/min. This supports the hypothesis that the missing H\({}_{2}\) in the tests with air injections has been combusted and converted into additional H\({}_{2}\)O, as confirmed by the consistency of the reported values of molar flows. In all of the tests, the H\({}_{2}\)S content was approximately 400 ppm on a dry N\({}_{2}\)-free basis. The NH\({}_{3}\) content, favored by the presence of steam as a gasification agent [32], was around 1000 ppm, with lower values at 850 \({}^{\circ}\)C. The higher NH\({}_{3}\) content at 800 \({}^{\circ}\)C, noticed both in the tests with and without air injections, was also observed by [PERSON] at al. in gasification experiments on coal macroals [33], and could be related to the combination of two effects: the increase of NH\({}_{3}\) production from the N-containing structures in coal enhanced in steam gasification with a higher temperature [32; 34; 35], and the thermal decomposition of NH\({}_{3}\) 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 O\({}_{2}\) increases the possibility of NH\({}_{3}\) 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 effect 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 N\({}_{2}\) in the air injected, which dilutes the O\({}_{2}\), could be the cause of the attenuation of the temperature increase due to the combustion, thus reducing the beneficial effect 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.