*2.1. Characteristics of Gasified Feedstock*

For all testing, conventional wood chips from alder trees obtained from local wood mills were used. For adsorption tests, the wood chips were not dried or sieved before feeding into the gasifier. The particle size of the chips was consistent (ca. 20 × 20 × 8). Their moisture content was checked regularly. Due to similar storage conditions, the fuel was air-dried and had on average 18% of moisture content in the working state. Table 1 presented below summarizes the proximate and ultimate analysis of the fuel (analytical state). Later in the study, experiments connected with the stability of filtration were also conducted, in which the moisture content of the feedstock was regulated.

**Table 1.** Proximate and ultimate analysis of wood chips gasified in GazEla reactor (a—analytical state, ar—as received = working state).


### *2.2. Chemical Composition of Analyzed Sorbents*

Four groups of naturally occurring minerals were selected for testing. The first adsorbent was chalk, which is rich in calcium carbonate. This material is widely available and used for the protection of filter elements (pre-coat), as well as a sorbent in high- and low-temperature cleaning of flue gases from combustion plants. It consists of ca. 94 wt.% clean CaCO3, which, under the filter conditions, undergoes calcination to CaO. The calcined form actively takes part in the adsorption of acidic compounds. However, it is also suggested that CaO can take an active role in catalytic recombination and degradation of tar components [14]. Chalk is used here as a representative of the Ca-rich family of sorption materials. The fine chalk for testing was obtained from LabTar sp. z o.o., and the product is commonly available on the market as a dietary additive for cattle.

The next two sorbents used in the research are closely related because they are derived from a similar family of sedimentary rocks. In nature, they are mostly distinguished by their tertiary structure and content of impurities. Thus, the second applied sorbent was halloysite. It is widely applied in the industry as a sorbent for petrochemicals and adapts a form of hollow tubes, similar to nanotubes. It is the form of halloysite that predisposes it for adsorption purposes. Due to these characteristics, halloysite also finds use as a feedstock for ceramic industry, as a dietary supplement for cattle, as a catalyst, and as a material for FB. The third sorbent was kaolinite, which mostly finds its application as a basic material for ceramic masses for the production of roof tiles, in chemical or paper industries, as well as in cosmetics. It mostly takes up the form of flat wafers of rhombic structure. From the chemical standpoint, both are hydrated alumina silicate and can have the following formula: Al4[Si4O10](OH)8·4H2O. Both were obtained from mines located in the southwest regions of Poland (Bolesławiec). In Poland, calcinated, fine halloysite is a market product of Intermark sp. z o.o., while kaolinite is marketed by Surmin-Kaolin S.A.

The last analyzed sorbent was dolomite, which is found in nature as a sedimentary, carbonate rock. In composition, dolomite mostly consists of a mixture of Ca and Mg (ca. 90 wt.%); thus, in this research, it was used as received for its Mg content, to judge its a ffinity for sorption processes. Similarly to chalk, fine dolomite used in the study is a commonly available fertilizer product manufactured by ZChSiarkopol sp. z o.o.

Even though, in filtration process conditions, all of the sorbents should take up their calcined forms, during the research, the sorbents were pre-calcined.

Regarding the handling of the sorbents, kaolinite showed a tendency to agglomerate when its moisture content increased even in an air-dried state. The same phenomenon was noticed for chalk, although to a smaller extent. The sorbent which by far was the easiest to flow and handle was dolomite, followed by halloysite.

Tables 2 and 3 presented below summarize the content of AAEMs and heavy metals in gasified biomass, as well as in sorbents, used for the gas cleaning research. As the adsorption tests were done in the order starting from chalk (T1), through halloysite (T2), kaolinite (T3), and ending up with dolomite (T4), the same numbering is used in the article to present the obtained data. For halloysite, two additional tests were also performed to assess its ability to stabilize filtration/regeneration cycles. The two are, thus, abbreviated T2.1 and T2.2.


**Table 2.** Overview of the chemical composition of alkali and alkali earth metals (AAEMs) present in the gasified fuel and applied sorbents.

**Table 3.** Overview of the content of heavy metals present in the applied sorbents (d—dry state).


Due to the unknown degree of volatilization of heavy metals from raw biomass, for determination of their content in filter cake, a separate analysis of filter cake obtained from sole gasification and HT filtration of the char/ash was performed. Here, no sorbent was used to coat the filters or to support the filtration process.

Figure 1 presented below compares ratios of the main metallic constituents and carbon build-up of the sorbents and biomass char. Evident differences in the chemical composition of the sorbents should impact their gas cleaning properties as Al, Mg, and Ca all take part in the sorption of acidic contaminants. Char, on the other hand, is particularly important in the readsorption of compounds volatilized from biomass (Ca, K, Mg) as it has a highly developed surface, similar to activated carbons. Char is also known to show some degree of catalytic activity for the reduction of tars.

**Figure 1.** Comparison of the chemical composition of sorbents and biomass char. Content of main constituents (inorganic elements and carbon) that actively take part in the adsorption process.

In the study, all laboratory analyses of sorbents, biomass, and gasification products were done at the Institute for Chemical Processing of Coal.
