**6. Conclusions**

To summarize, during the research, the use of four sorbents of distinctive chemical and physical properties was tested for HT conditioning of syngas, namely, chalk (CaO), dolomite (CaO–MgO), halloysite (AlO–SiO), and kaolinite (AlO–SiO). The two representative aluminosilicates differed from each other in their tertiary structure.

The research showed that co-filtration of dolomite with solids from the gasification process may inversely affect the regeneration of HT filters but lead to quick and irreversible filter failure. Even though the sorbent itself is easily flowable, is easy to handle, effectively pre-coats the filters, and gives good sorption parameters, it is also the first material on the list of tested sorbents which cannot be reported to have filtration-enhancing properties until further research on merits of its application is available.

For the rest of the tested sorbents, design criteria regarding both pulse-back regeneration of the filter and sorption of syngas were satisfied. Collected data allowed concluding that the concept for one-vessel multicomponent gas cleaning is possible and can be successful.

All tested materials adsorbed 90%–99% of Cl present in raw syngas, thus reducing its content to levels below 5 ppmv, which, for most energetic applications, is considered as acceptable.

HT filtration and sorption provides a means for the reduction of high-molecular-weight tar content in the range of 10% to 18%, depending on the sorbent used. From preliminary results, it was found that CaO- and CaO–MgO-based materials have higher tar reforming capabilities.

In pilot-scale gasification process conditions, no direct sign of preferential removal of AAEMs on the filter cake-containing sorbents was found. Char in the fixed bed acts as the first stage adsorber and causes retention of compounds volatilized from ash, leading to the high variability of the obtained results.

After filtration on precoated ceramic filters, no presence of PM could be measured in the pilot-scale gasification installation. The only source of solids determined in gas after filtration originated from a temporary breakthrough of particles after pulse-back cleaning or marginal condensation and polymerization of heavy tars.

The application of high-e fficiency, HT filter for dedusting of syngas was proven to be the preferred technology for the first step of syngas cleaning. Dust-free syngas allows for its direct cooling and condensation of organic matter, as well as cleaning in oil scrubbers or its further upgrading without problems related to fouling of the installation. In syngas cleaning, however, it is vital to remove tars as quickly as possible and accordingly to their condensation temperature. The fouling of heat exchangers in dust-free syngas conditions follows the path of condensation of heavy tars on contact with "cold tubes" of the heat exchanger and further polymerization of the liquefied tars. A solution to this problem was found in all applications where syngas cooling is either more rapid and goes down to temperatures below the water dew point or does not cool syngas down to levels where heavy tars can condense.

Furthermore, detection of H2S was expected during ion chromatography of the condensed phase of syngas, as well as from the dedicated sampling of syngas through NaOH absorption. However, for all tested samples, no S ions were measured. This result indicates that the applied analytical procedure needs to be further developed in the future to give better precision in the determination of syngas composition.

Finally, from the analysis of obtained data, halloysite was shown to give the best overall performance in syngas cleaning through sorption-enhanced HT filtration. It had superior filtration and pulse-back cleaning properties, and it remained flowable in all apparatuses used for its storage, handling, and feeding. For very high filtration velocity (1.9 m/s) and syngas of high water and heavy tar content, it allowed the filtration to remain stable even though the set point of dP had to be increased to 2 kPa. In terms of gas cleaning properties, CaO remains the best solution because of its higher tar reforming properties and the comparable ability for the removal of Cl.
