**1. Introduction**

For most small- and medium-scale applications of gasification technology, the gasified feedstock is either bio-based or, more preferably, derived from waste feedstocks such as solid recovered fuel (SRF), lignin, or sewage sludge [1,2]. In the case of most waste-derived feedstocks, the step of their thermochemical conversion is more demanding than commonly encountered for conventional, clean biomass. The problems are mostly encountered in the operation of reactors; however, the use of waste materials also introduces major changes in the way the syngas needs to be purified. Importantly though, where the control and efficiency of a gasifier is the most important aspect for the operation of any gasification installation, it is the possibility to clean the produced syngas that renders the whole operation economically and environmentally sound. With the increasing complexity of syngas application, the complexity of its purification also increases. In general, direct combustion requires only marginal gas cleaning, which can often be limited merely to dedusting in cyclones, while, for reciprocating or gas-turbine engines, complete removal of fines, deep removal of condensing species (tars, light organics, and water), and careful control of the content of acidic species are demanded. By far, the most advanced applications of syngas are chemical synthesis (methanation, Fisher-Tropsch) and fuel cells, both of which demand almost complete removal of any contaminants [3]. It is compulsory, then, to devise such gas cleaning systems so that three major goals are simultaneously obtained. The first is to achieve the required cleanliness of the gas. The second is to perform the process with the least environmental impact, while the third connects with the economic aspect of the investment. Thus, the third issue not being technologically limiting remains crucial when developing new technologies.

Due to the progress achieved in gasification and gas cleaning technologies over the past 30 years, today, we most often see that it is the economy that holds back the development of many gasification installations rather than their technical or technological limitations. The fact is that, depending on a given case, from technological and technical standpoints, different feedstocks perform better in different types of reactors. Thus, the choice of gasifier can determine the whole installation's success or failure. Furthermore, gas cleaning units are developed for a given gasifier and feedstock, not vice versa. It is also well established that gas cleaning with a combination of the following methods provides the best performance in the widest area of syngas cleaning applications: catalytic upgrading, high-temperature (HT) dedusting on barrier filters, wet scrubbing, and final polishing with dry adsorption methods [4]. These advanced solutions are compulsory for high-value end products obtained from syngas. However, at the same time, they tend to lower the profitability of installations in applications where low-value products, such as heat and power, are produced. Currently, in many developing countries such as Poland, it is the eco-friendly, renewable generation of electrical power that shapes the scene in terms of political decision-making and market development initiatives much more firmly than the drive toward, e.g., production of liquid biofuels. To make progress in the amount of waste biomass, SRF, or sewage sludge utilized for power generation, it is vital then to take advantage of small- and medium-scale distributed gasification installations, which o ffer better e fficiencies, flexibilities, costs, and environmental impact than conventional combustion methods. For these reasons, development of gas cleaning methods that provide the above-mentioned benefits needs to be done.

The article describes the concept of an adsorption technology tested for HT cleaning of syngas derived from the gasification of biomass. The concept is based on the use of a multi-component cleaning method performed in one reactor vessel, where a hot-gas ceramic candle filter is precoated with naturally occurring cheap and abundant minerals. Multi-component cleaning solutions greatly improve the reliability and e fficiency of thermochemical conversion plants, while also lowering their footprint. HT, raw syngas coming out from a reactor contains the highest amount of contaminants, which makes its cleaning most e fficient. Furthermore, because HT dedusting is most of the time the first step of syngas cleaning, it is interesting to develop a method for simultaneous removal of not only dust particles but also other contaminants. For this reason, it is proposed to inject adsorbents upstream of ceramic filters and, thus, to conduct the adsorption process on a continuously regenerated bed of filter cake composed of char, ash, and the adsorbent material itself. This concept has multiple potential merits. The most important ones are the complete removal of solids and the possibility to reduce the amount of Cl, S, and tar species.

Firstly, it is noteworthy that, after the gasification process, both chars and ashes entrained in syngas have substantially activated surface area and, thus, can act as a sorbent for contaminants. This phenomenon is currently thoroughly researched since major improvements in fixed-bed gasifiers are possible this way. Nakamura et al. [4] proposed the use of gasification by-products as a means for process gas cleaning. In the system, the use of water–tar condensate as a washing medium in a scrubber and char in the form of a fixed-bed adsorber was tested. The results showed that the scrubber e fficiency for tar removal reached its optimum at 50%; however, the e fficiency of the char adsorber reached over 81%. Furthermore, Yafei et al., in their work, reviewed and studied the concept of closing the managemen<sup>t</sup> cycle for tar, char, and heavy-metal gasification by-products through integrated concepts [5]. In the study, the pathway for using gasification- or pyrolysis-based chars as support for adsorption of heavy metals and their subsequent use (after deactivation) for catalytic elimination of tars was presented. Finally, the closure of the tar/char managemen<sup>t</sup> cycle was proposed through the use of catalytic gasification of the spent catalyst. In this way, it is possible to recover C from the catalyst as syngas and the heavy metals as bottom ash. However, the most straightforward and visual way of using creative design of thermochemical processes to use the characteristics of gasification char was presented by Obernberger et al. [6]. The study showed that it is possible to use a two-stage gasification–combustion method for the energetic use of waste biomasses of low ash melting temperatures for highly e fficient generation of heat with unprecedentedly low emissions of CO, total organic carbon, and particulate matter. Here, the process layout resembles a common updraft gasifier with a gas combustion chamber that is fitted just above the fixed bed. However, the obtained in-bed temperature profile is distinctly di fferent for the char/biomass bed to act as adsorber/filter and not to produce excessive amounts of tars, as is often encountered in conventional updraft gasifiers.

Furthermore, process conditions where most syngas ceramic filters are commonly operated induce the need to use auxiliary co-filtering materials in the first place. The inert materials increase the filter's particle collection e fficiency, as well as keeping the pressure-drop low. Such inerts are mostly based on fine powders composed of SiO or CaO. Through this "co-filtration" e ffect, it is often possible to keep dedusting of hot syngases stable, where filtration of char alone leads to a constant rise of pressure drop on the filter. Due to its higher thermal stability and flowability, the inert material eases pulse-back regeneration of the filter, while collecting part of the polymerizing tar and condensing mineral matter, thus preventing the clogging of filter pores. By changing the inert solid for materials that exhibit chemical activity in the process condition, it is possible to perform high-temperature, dry scrubbing of syngas.

In the proposed system, the sorbent is injected upstream of the ceramic filter through a Venturi nozzle. This method assures good mixing of the solid and syngas in the turbulent region of the filter intake. In the first step, the adsorption takes place in a diluted two-phase system, where contact time is extended by the design of the dirty plenum of the filter. Importantly, the particle size distribution of the sorbents needs to be controlled to avoid disengagement of the sorbent from syngas before reaching the surface of the filter cake. The adsorption process is finalized in thorough cleaning on a fixed-bed layer composed of the filter cake which collects on the surface of the ceramic filters. When adsorption on the filter cake is considered, a few di fferences render the process distinctive from a conventional adsorption set-up. Firstly, filters applied in the filtration of gases are mostly developed to operate at as high linear gas velocities as possible (face velocity). However, in HT applications, to keep them stable, the units need to be run in the range of face velocities much lower than nominal for low-temperature bag filters. Values from 0.5 to 3.0 cm/s often provide the lowest operational and investment costs for hot gas filters, while keeping the process stable. In adsorption processes, hourly spaced gas velocity (HSGV) can be thought of as a parameter comparable to face velocity in filters because, here, the filter cake is the active bed composed of the sorbent material. For adsorption to be e fficient, the HSGV in fixed-beds should be kept in the range from 0.2 to 0.5 m/s with a contact time of approximately 3 s [7]. To ge<sup>t</sup> close to the above-mentioned standards, it is necessary to lower the filtration velocity to minimal values and increase the thickness of filter cake. In practice, reaching the benchmark with the use of ceramic filters is technically impossible.

For the pilot installation used in the research, the ceramic filter integrated with the pilot GazEla gas generator was designed to be operated at 0.5 to 1.0 cm/s. Depending on the operational conditions of the filter, the thickness of its filter cake should range from 1 mm for freshly pre-coated new filters up to 6 to 8 mm when the filter is run with candles not regenerated regularly. Realistically speaking, this set-up may provide from a minimum of 0.2 s up to a maximum of 1 s of contact time between the gas phase and solid phase of the filter cake with an average of 0.5 s. Such residence times should be considered as low values for fixed-bed adsorbers; however, they are more than reasonable for many fluidized bed (FB) units [7]. In the proposed system, the first stage of adsorption resembles FB adsorption in a diluted circulating fluidized bed (CFB), as the adsorption process starts from the moment the sorbent is injected into syngas. For the pilot installation, the contact time for the first stage of adsorption can reach up to a few seconds depending on the reactor power.

The second reason why the HT filtration/adsorption system cannot be directly compared to any of the two above-mentioned systems is the particle size of sorbents that build up the filter cake. Preferably, for the filter, the sorbent particle size distribution should range from 15 to 50 μm, which makes it 10-fold smaller than sorbents used in FB reactors and more than 1000-fold smaller in comparison to fixed-bed units.

Moreover, it is important that, after a regeneration cycle takes place, part of the surface of the filters is stripped of the filter cake. If the regeneration is too harsh, the thickness of the cake may be reduced to the point where both particles and other contaminants slip through the filter, thus impairing the e fficiency of cleaning. Furthermore, as mentioned above, the filter cake only partially consists of the dedicated adsorbent, whereas the rest of the cake is composed of char and ash filtered from

the syngas. Both ash and char are transported through a gasifier; thus, their surface is activated to some extent, which has a positive e ffect on the adsorption e fficiency. Sorption in the filter cake also brings forward another relation. The thickness of the cake positively influences the e fficiencies of both dedusting and adsorption, while increasing pressure drop (dP) across the vessel. Thus, the importance of precise control of the regeneration process is vital here.

Research presented in this paper focused on the collection of process data and operational experience. However, to assess the e fficiency of the method, it also leaned into the determination of adequate analytical procedures. In contrast to flue gas cleaning systems, the problem of qualitative and quantitative determination of cleaning e fficiencies of syngas cleaning units is still demanding and by no means should be regarded as trivial. This issue is even more pronounced when research is performed in real conditions and on a small scale, which brings forward many technical limitations that are not present in lab-scale installations.

The extent to which the syngas needs to undergo cleaning is determined by the type of feedstock, the type of gasification reactor, and the of the final application. Generally speaking, when the reactor is treated as an equilibrium black box, its products are always the same and depend only on process efficiency, temperature, pressure, type of gasifying agen<sup>t</sup> used, etc. Thus, models often do not take into account the recognition of the type of gasifier used. However, in reality, di fferences between the gasification processes run at fixed, fluidized, or entrained beds are substantial and decide the composition of generated syngas. As far as contaminants of syngas are concerned, their generation in gasifiers takes place mainly due to volatilization from the solid phase and subsequent gas–gas and gas–solid reactions. Due to di fferent process conditions taking place in gasifiers, di fferences in syngas composition are pronounced. For fixed-bed reactors, a readsorption of contaminants on activated char and ash present in the bed is observed [8]. For FB, on the other hand, the amount of char in the bed is small, but the bed is often composed of a material which can influence the process through catalysis or sorption. For example, in FB reactors, olivine is a frequently used as a bed material for in situ tar reforming, and CaO beds are used for CO2 removal from syngas [9,10]. Finally, entrained flow reactors are operated at very high temperatures that can lead to very clean syngases through the almost complete conversion of tars and collection of containments in the form of vitrified slag [11]. For the Institute for Chemical Processing of Coal (IChPW), the choice of fixed-bed reactor systems was done based on the scale of solution sought by the market and the intrinsic characteristics of fixed-bed units, meaning their ease of operation, robustness, flexibility, and ability to use the bed for active removal of contaminants from syngas. In fixed-bed reactors, char is activated by process conditions and acts as the sorbent material. For example, in laboratory conditions, it was shown that, inside gasifiers, chlorine can be almost fully volatilized (>90%) from fuel and, thus, is a constituent of raw syngas. In the case of S, it rarely undergoes complete volatilization in the reactor and, on average, 50% of S from biomass remains in the bottom ash from the reactor even at temperatures reaching 1000 ◦C [12]. On the other hand, alkalis, such as potassium, almost exclusively volatilize with correlation to Cl. Therefore, if Cl content of the biomass is low, K mostly remains in the bottom ash. Other elements that tend to volatilize under gasification conditions are mainly Na, Ca, Si, Mg, P, and Al; thus, in this research, their content in the filter cake was the subject of examination. Okuno et al. [13] suggested that alkali and alkali earth metals (AAEM) leave gasifier mostly in correlation with water-insoluble tar and, thus, with aromatic compounds that are derivatives of benzene, xylene, furfural, and naphthalene in the syngas [14,15]. These compounds remain gaseous under gasification and HT gas cleaning conditions; hence, their enhanced recovery in filer cake is a sign of adsorption from the gas phase. Moreover, Sonoyama et al. [16] showed that AAEM species have high a ffinity to bonding with char in fixed-bed conditions unless the flow of gas is significant enough to force them to pass through the bed. In their research, helium was used as the carrier gas; however, the principle also applies to the research presented in this paper. These results showed that volatile AAEM species undergo repeated adsorption/desorption cycles on the surface of the char bed; thus, their readsorption on filter cake should also be visible. In the study, the authors experimentally verify the theoretical possibility of removing the following contaminants from syngas with the use of a combined process of HT filtration and sorption: hydrogen sulfide, hydrogen halides, ammonia, AAEMs, heavy metals, and tars on the surface particles collected in the filter cake.
