*2.1. Materials and Reagents*

The hydroxypropyl methylcellulose brand was Walocel™, produced by Dow Corning (Dow Corning Inc., Midland, MI, USA). Hydroxypropyl methylcellulose is a natural polymer that dissolves easily and quickly in hot or cold water, forming solutions with different viscosity levels.

The sodium alginate was produced by Qingdao Yingfei Chemical Co (Qingdao Yingfei Chemical Co. Ltd., Shandong, Qingdao, China). Sodium alginate is a linear polysaccharide derivative of alginic acid comprised of 1,4-β-d-mannuronic (M) and α-l-guluronic (G) acids. Humic acids obtained during the extraction of lignite with an alkaline solution of sodium pyrophosphate followed by extraction with a 1% solution of sodium hydroxide and precipitation with mineral acid were used as hybrid modifiers. Table 1 shows the characteristics of the humic acids.

**Table 1.** Proximate analysis of lignite \*.


\* Wa—moisture in the analytical state; Ad—ash content in a dry state; S<sup>d</sup> <sup>t</sup> (Sdaf <sup>t</sup>)—total sulfur content in a dry state (per organic mass); Vdaf (V<sup>d</sup> )—yield of volatile substances per organic mass (in a dry state).
