*4.1. Generation of Syngas*

During all sorption tests, the reactor was operated on the same feedstock and its parameters were set to the same limits, so that the stream of syngas, its composition, the temperature, and the amount of impurities would be comparable. Start-up was performed after initial preheating of the reactor lining to ca. 350–450 ◦C. At this point, the process bed was formed and ignited by the addition of a hot air/steam mixture. The stream of the gasification agen<sup>t</sup> was controlled and steadily increased up to the full capacity of the reactor. Until the bed reached its final temperature profile, and as long as the temperature of syngas at the reactor's outlet was not stable, the gas was directed to a burner without any gas cleaning. When the reactor reached stable operation (ca. 2 h), syngas was fed into the gas cleaning installation. After the installation reached stabilization (another 2 h), the sampling for syngas cleaning efficiencies was started.

### *4.2. Filtration Process and Filter Regeneration: Measurement and Method for Analysis of Data*

The ceramic filter was continuously monitored with a range of temperature and pressure readings. The two most important parameters for its stable operation are readings of gas temperature at its inlet and outlet. It is known that the origin of syngas greatly influences the range of operational temperature for HT filtration (operational window). If a certain temperature limit is exceeded, filtration may lead

to a complete failure of the filter as a result of a continuous increase of di fferential pressure (dP) on the filter. In such a case, the filter cake often undergoes some degree of sintering or obtains some degree of viscosity, both of which result in greater resistance for pulse-back cleaning and render the regeneration ine fficient. Above the upper-temperature limit, the filter pores can become plugged as a result of condensation of mineral matter volatilized from fuel and chemical activity of tar, which leads to their polymerization and char formation. The lower limit, on the other hand, protects from the condensation of high-molecular-weight tars within filter pores. For this reactor and conventional biomasses, the operational window of HT filtration should be kept in the range of 350–500 ◦C.

The most important control parameter for ceramic filters is its di fferential pressure (dP) across the filters. The measured dP summarizes both the dP generated by filter media and the filter cake collected on it. As the amount of solids collected in the filter cake or its compressibility increases, so does its flow resistance and, thus, dP of the filter increases. At a given set point, the filter is subjected to pulse-back cleaning to recover a desirable low dP. To keep operational costs of gasification installation as low as possible, filters should be run at the smallest possible dP. In the long term, stable and reliable operation of HT filters at low dP is obtained by performing the filtration mostly as depth filtration within the volume of a filter cake. In this way, the filer medium does not become irreversibly blocked by dust particles and the e fficiency of collection is kept high. For this unit, such results are attainable when it is operated at a dP of 1–2 kPa. Above these values, a build-up of excessive filter cake can lead to failure of the filtration process, e.g., as a result of filter cake bridging.

For a more precise comparison of di fferent tests and standardization of analyzed data, a convention was set here, where the registered pressure drop across the filter is divided by the actual flow rate of gas through the filter.

During the research, a standardized procedure for the preparation of the filter candles and their pulse-back regeneration was set. Filters before each test were thoroughly cleaned through pressure pulses when the installation was o ffline (20 ◦C with a full reverse flow). After that, at process temperatures (ca. 450 ◦C), candles were pre-coated by a controlled layer of the tested sorbent. During a test, filtration was started when the reactor reached stable operational conditions and maintained them for a minimum of 2 h, whereby the temperature of gas exceeded 450 ◦C and its stream was constant. From the beginning of filtration, gas was injected with a sorbent, and, throughout the test, the stream of the sorbent was kept constant. Regeneration cycles were controlled automatically by system control and data acquisition (SCADA) and for the normal, continuous operation started at dP = 1 kPa.

### *4.3. Methods for Measurement of Contaminants Present in Syngas*

Another aspect of the development of a new syngas cleaning method is to establish a methodology for the determination of its performance parameters. There exist standardized analytical procedures and sampling methods for the analysis of syngas composition and the amount of contaminants contained in it. For organic matter in the gas, a well-established standard set in the "Tar Protocol" was applied for years. The protocol proposes sorption in isopropyl alcohol or on solid sorbents as the best methods for the collection of organics from syngas. However, even though this method is applied globally for a long time now, the variety of results obtained from similar reactors and process conditions show that there still is a gap in the analytical procedures. The measurement conditions that occur in raw syngas increase the measurement error dramatically. To try and address this issue, many di fferent approaches were proposed. One of the most resilient ideas for determination of the e fficiency of syngas cleaning is an analysis of its combustion products through online measurement of the flue gas composition. This indirect method uses hot-sampling and FTIR analyzers, which give a very good range of measured compounds and very low detection levels for all contaminants in their oxidized forms. The drawback here is that post-combustion analysis gives information only about the overall efficiency of a gas cleaning system and it is impossible to point out the e fficiency of a given apparatus.

To obtain more direct and reliable data, a di fferent approach was adopted for this study. The first step of the method was a basic, on-line measurement of syngas composition using IR analyzers. This robust measurement provides on-line information regarding changes in the gasification process and quality of generated syngas. Tar, water, and particle matter contents were measured by taking samples of the gas in parallel, before and after the ceramic filter. The remainder of the analysis, i.e., removal of AAEMs, halides, and H2S, was determined from the analysis of liquid and solid products recovered from the gas cleaning units. Simultaneously, another analysis of halides and H2S was performed through their absorption from syngas (impinger bottles with NaOH).

The liquid samples were analyzed for the presence of halides and H2S.

### 4.3.1. Water, Tar, and Solid Particle Content

The tar content of syngas is inherently connected with the gasification process. Its content not only indicates the loss of conversion efficiency, but is also the most important issue that prevents larger market uptake and technical utilization of the gasification process.

The simultaneous filtration/sorption process is not devised to directly convert or reduce tars in syngas. However, there exist experimental results indicating that HT filters may induce a partial reduction of syngas tar content. In this research, an adapted method for the quantification of tars based on the Tar Protocol was used.

The sampling system consisted of a probe, two impinger bottles, and a tube filled with cotton wool. The probe was introduced axially into a syngas pipe. The end of the probe was connected to the first impinger bottle containing about 50 mL of isopropyl alcohol at ambient temperature. Here, most of the tar and dust was collected. Moreover, in this region, the syngas was also saturated with the solvent which prevented water from freezing further down the collection line. The first impinger bottle was connected with the rest via a Teflon tubing, which acts as an additional condenser. The second bottle, filled with 50 mL of isopropyl alcohol at −20 ◦C, collects the remaining water and low-boiling-point volatile organic carbon (VOC). Finally, the glass tube, which is filled with cotton wool, acts as a droplet collector. Producer gas is sampled through the system by a pump coupled with a flow meter and a regulator.

After sampling, both sorption solutions are combined and analyzed together for the content of volatile organic carbon (VOC), water, dust, and tars. In this research, no qualitative analysis of VOC was conducted.

The concentration of water in isopropyl alcohol solutions was determined by the Karl Fischer method with the use of a Mettler Toledo automatic titrator. Tars and solid particles were measured gravimetrically. The dust contained in isopropyl alcohol solutions was filtered <sup>o</sup>ff, washed with an additional portion of isopropyl alcohol, dried, and weighed. The mass of tar was measured after evaporation of the solvent under reduced pressure (0.1 bar, 80 ◦C) until a constant mass was reached.
