*5.3. Heavy Metals*

The starting assumption for the determination of collection e fficiency of heavy metals is that, in process conditions of the filter, the species that contain them are attached to other solids present in the gas. When subjected to filtering, they should be recovered either directly through filtration (mechanical separation) or indirectly by adsorbing onto the surface of filter cake particles, as they already have a well-developed surface and the HT filter conditions are more favorable for adsorption than those that occur in the reactor. Table 7 collates the obtained results regarding the amount of heavy metals present in fresh sorbents, as well as in filter cakes recovered after the filtration/sorption process. Values presented in parentheses in each cell correspond to the given species content of the fresh sorbent.


**Table 7.** Concentration of heavy metals in samples of feedstocks and products from gasification and filtration/sorption processes (unit: mg/kg on a dry basis).

In the case of five elements, a 10-fold increase in concentration was registered in comparison to the result of the fresh sorbent. These elements were Cr, Mn, Mo, Ni, and Zn. In the case of Zn and Mo, an increase in concentration in all samples of filter cakes was noted. For Zn, the highest gain was noted in T3 (25-fold for kaolinite), while the lowest was noted in T4 (six-fold for dolomite). Previous studies on the volatilization of metal compounds during the thermochemical treatment of ashes concur with this finding [12–14]. For Mo, higher recovery was noted in T1 (17-fold for chalk), while the lowest was noted for both T2 and T3 (two-fold for halloysite and kaolinite). For Cr, a higher increase in concentration was noted in the case of T1 (25-fold for chalk), while the lowest was noted for T3 (two-fold kaolinite). For Mn, T4 gave the highest increase in concentration (840-fold for dolomite), while the lowest was noted for T3 (3.6-fold for kaolinite). The increase in the amount on Mn in the T4 filter cake was substantial, and the determination of its origin needs further investigation. For Ni, the highest rise in concentration was noted for T1 (23-fold for chalk), whilst the lowest was noted for T3 (3.5-fold for kaolinite).

Thus, all sorbents showed the concentration of heavy metals in their relevant filter cakes higher than expected from mass balances. During the research, there was no method for the determination of any residual heavy metals in syngas downstream of the ceramic filter that could support any definitive conclusions on any preferential removal of heavy metals.
