Table 3.

Research status of coal gangue and fly ash in the field of energy regeneration.

Regenerate Objects	Regeneration Effect	Catalytic Material	Preparation Method	Action Mechanism
Waste polyethylene (PE)	Reaction temperature: 700 °C, ratio of feed to catalyst: 20:1, maximum liquid product yield: 78.2%, with total aromatics yield reaching nearly 22%.	Modified fly ash	The coal fly ash was crushed, sieved, and heat-treated at 800 °C to modify [88].	The fly ash catalyst with high specific surface area and high Si/Al ratio provides a large number of highly selective acidic sites, promoting the cracking of plastics and improving the yield of aromatics [89,90].
Waste linear low-density polyethylene (LLDPE)	Catalyst dosage: 15 wt%, LLDPE’s cracking activation energy significantly decreased, and reaction rate and the yield of alicyclic hydrocarbons increased.	Modified fly ash	The coal fly ash was ground to a particle size of less than 150 μm to modify [91].	Solid alkali components such as CaO in fly ash facilitate the formation of carbon anion intermediates, promoting the generation of alicyclic hydrocarbons and accelerating the reaction rate [92]. Transition metal oxide components such as Fe2O3 in fly ash also assist the plastic cracking process.
Plastic film residue (PFR)	Reaction temperature: 723 K. Use of catalyst inhibits tar and wax formation, reduces cracking temperature, and achieves a 44% product oil yield, with 70% composed of gasoline hydrocarbons.	HX/CFA zeolite	Coal fly ash (CFA) underwent alkali fusion and hydrothermal synthesis to produce NaX zeolite. After acidification, HX/CFA zeolite was obtained [93].	Acidic sites on synthesized X-type zeolites (solid acid) promote the formation of carbocation intermediates in polymer chains, enabling high-quality and efficient cracking of waste plastics [89,90].
Waste wood pellets	A 10 wt% Ni-loaded catalyst and waste wood pellets were steam reformed to obtain a 54.9% gas yield, with an H2 production of 7.29 mmol/g.	Ni-ash catalysts	Coal fly ash was impregnated in a Ni(NO3)2 · 6H2O aqueous solution after drying, calcining, reducing it in H2 atmosphere to prepare the Ni-ash catalyst [94].	The hydrogenation and dehydrogenation functions of metal Ni, as well as the isomerization, cyclization, and hydrogenation cracking functions of acid components (Al2O3) in fly ash carrier occur through olefin intermediates during steam reforming. Metals such as Mg and Cu in fly ash also act as catalyst auxiliaries.
Pine sawdust biomass tar	Reaction temperature: 800 °C. The catalyst effectively promoted the decomposition of tar molecules, achieving a conversion rate of 93.5%, and significantly increasing the gas yield.	GC catalysts	The coal gangue (GC) was crushed and sieved, and then calcined at 800 °C for 1 h under N2 atmosphere to obtain the GC catalyst [95].	The repeated oxidation and reduction of Fe2O3 in coal gangue effectively promotes the breaks of C-H bonds and dehydrogenation reactions in catalytic reactions, increasing the yield of H2 and CO. Alkali metals in coal gangue further facilitate tar decomposition. The formation of Fe0 during the process enhances the activity and lifespan of the catalyst [96,97].
Soybean oil	Reaction temperature: 65 °C, catalyst concentration: 4%, molar ratio of methanol to oil: 12:1, reaction time: 2 h. The conversion rate of soybean oil methyl ester reached 95.5%.	Zeolite-type sodalite	Coal fly ash was added to an alkali solution and supplemented with an aluminate solution to form a gel. The zeolite-type sodalite was formed through hydrothermal crystallization at 100 °C for 24 h using the gel [98].	Solid alkali components (Si-O-Na groups) in zeolite serve as active sites, promoting transesterification to produce biodiesel.
Jatropha curcas oil	Catalyst (40 wt% CaO loaded) dosage: 0.15 wt%, molar ratio of methanol to oil: 12:1, reaction temperature: 60 °C, and reaction time: 1 h. The maximum biodiesel yield was 94.72%.	CaO/CFA catalysts	Coal fly ash was activated by calcination at 400 °C for 5 h, impregnated with a calcium acetate solution and dried, then calcined at 750 °C for 4 h to obtain CaO/CFA catalyst [99].	The appropriate amount of CaO loading improves the catalyst’s pore structure and increases the reaction rate. The introduced CaO (solid alkali) serves as an active site for transesterification, promoting the formation of biodiesel. Si and Al in fly ash act as catalyst carriers to improve catalyst stability.
CO and H2	Reaction temperature: 523 K, air pressure: 20 bar, and H2/CO = 2. Zeolites carrying more Co particles have higher catalytic activity, resulting in a CO conversion rate of 53.2% and higher selectivity for producing liquid hydrocarbons (C5+).	Co/LTA and Co/FAU catalysts	LTA and FAU zeolites synthesized from high-silicon coal fly ash were used as carriers. The Co/LTA and Co/FAU catalysts were obtained by impregnating with Co(NO3)2 solutions and calcination in H2 atmosphere [100].	CO and H2 are adsorbed on the surface of metal Co, activated through electron effects and interactions with transition metals, and then dissociated, leading to chain growth and termination to produce various hydrocarbons [101]. Fly-ash-based zeolite carriers plays a role in improving pore structure, enhancing catalyst stability, and increasing the activity of active components during the process.
CO and H2	Reaction temperature: 220 °C and air pressure: 30 bar for FT synthesis. After 130 h of intake, the conversion rates of CO and H2 were 67% and 73%, respectively. The selectivity for liquid hydrocarbons (C5+) was 66.53%. Among C5+ hydrocarbons, the contents of gasoline (C5–C11), diesel (C12–C18), and high-carbon hydrocarbons (C19+) were 33.19%, 27.64%, and 5.7%, respectively.	Co-Fe/SBA-15 catalysts	Coal fly ash was activated by alkali fusion. A small amount of surfactant was added to hydrothermally synthesize mesoporous SBA-15 zeolite. The Co-Fe/SBA-15 catalyst was prepared by impregnating with Co(NO3)2 and Fe(NO3)3 solutions and calcining in H2 atmosphere [102].	The loading of additional Fe in addition to Co makes the catalyst more advantageous for synthesizing low-carbon hydrocarbons, promoting the generation of gasoline-range products. This catalyst also has a wider CO/H2 range and higher poison resistance during F-T synthesis.