Dry anaerobic digestion (DAD) is an attractive method for simultaneous organic waste disposal and bioenergy recovery. DAD has the problems of low methane yields, low reaction rates, and easy inhibition due to its limited mass transfer and heat transfer. In this work, two methods of mechanical stirring and percolate recirculation were compared regarding their capacities of improving the mass transfer and enhancing the performances of DAD in batch experiments with sorghum stalks as a substrate. The cumulative biogas yield and system stability were investigated when the stirring linear velocity was 0 cm/s, 22 cm/s, 44 cm/s, 66 cm/s, and 88 cm/s. When the stirring linear velocity was 88 cm/s, the cumulative biogas yield and methane content were highest. The computational fluid dynamics (CFD) simulation indicated that the shearing force near the stirring shaft was largest. When the linear velocity of the stirring paddle was 88 cm/s, the shearing force at a radial distance close to center was about −140 N/m². When the ratio of the material stacking height to the reactor diameter (H/D) was 3:2, the AD showed the best performance. A higher material stacking height promoted the contact between the microorganisms and the substrate and enhanced the biogas production. By combining percolate recirculation and mechanical stirring, the cumulative biogas yield increased by 28% compared with the static DAD process because of the promotion of mass transfer in the DAD.

To prevent liquid leakage during the phase transition of a phase change material (PCM), a novel form-stable PCM (FSPCM) based on LA/CIT/CNT was fabricated using a simple and facile direct impregnation method. The iron tailings (ITs) was calcinated at first. And then lauric acid (LA) was impregnated into the calcinated iron tailings (CITs) with carbon nanotubes (CNTs) as a thermal conductivity additive. Subsequently, the leakage tests and the properties of the prepared samples were investigated by diffusion-oozing testing (DOT), SEM, XRD, FTIR, DSC, TGA, and intelligent paperless recorder (IPR). DOT results showed that the impregnation ratio of LA into the CIT and CNT was up to 27.5% without leakage. SEM indicated that LA can be adsorbed into microscale pores and covered the surface of CITs and CNTs. FTIR spectra indicated that there was no chemical reaction during the preparation process. The melting and freezing temperatures of the prepared LA/CIT/CNT FSPCMs were measured as 45.24 °C and 39.61 °C, respectively. Correspondingly, the latent heat values were determined as 39.95 J/g and 35.63 J/g, respectively. The LA/CIT/CNT FSPCMs exhibited good thermal stability in the working temperature range, and its heat transfer efficiency was improved significantly by 69.23% for LA and 84.62% for LA/CIT FSPCM. In short, LA/CIT/CNT FSPCMs are a very promising material for thermal energy storage in practical low-temperature applications.

Fly ash is a kind of industrial solid waste that is considered “hazardous waste”. In this study, a supporting matrix of modified fly ash (MFA) was employed to package lauric acid (LA) via a facile direct impregnation method involving less experimental error. A low-cost and eco-friendly form-stable phase change material (PCM) of LA/MFA/graphene (G) was fabricated, with G as the thermal conductivity enhancer. The preparation and leakage testing of an LA/MFA/G form-stable PCM (FSPCM) were investigated in detail. The leakage test results indicated that good package efficiency was obtained using MFA with a higher specific surface area and richer pore structure to pack the LA. Then, LA/MFA/G composites were characterized via scanning electronic microscope (SEM), Fourier transform infrared spectroscope (FTIR), differential scanning calorimeter (DSC), and thermal gravimetric analyzer (TGA). The results showed that excellent form stability was obtained by adding MFA as the supporting matrix. The SEM analysis indicated that LA could be well dispersed into the structure of MFA. The FTIR analysis demonstrated that the components of the FSPCM were quite compatible. The results of the DSC illustrated that LA/MFA/G (5 wt. %) had a melting point of 45.38 °C and a latent heat of 41.08 J/g. The TGA analysis revealed that the prepared FSPCM had better thermal stability compared with LA within its working temperature range. In addition, the effects of G on the heat transfer performance of the prepared FSPCM were examined. In short, using MFA with a higher specific surface area and richer pore structure to pack the LA via a simple preparation process with less experimental error can contribute to good performance. The research not only improved the comprehensive utilization of solid waste, but also promotes the application of FSPCM in the field of building energy conservation.

The influence of adding surfactants on the performance of high-solid anaerobic digestion of horticultural waste was extensively investigated in batch systems. Adding Tween series and polyethylene glycol series non-ionic surfactants had positive effects on biogas production, resulting in 370.1 mL/g VS and 256.6 mL/g VS with Tween 60 and polyethylene glycol 300 at a surfactant-to-grass mass ratio of 0.20, while the biogas production of anaerobic digestion without surfactants was 107.54 mL/g VS. The optimal and economically feasible choice was adding Tween 20 at a ratio of 0.08 g/g grass in high-solid anaerobic digestion. A kinetics model reliably represented the relationship between surfactant concentration and biogas production. The mechanism of surfactants working on lignocellulose was investigated. The improvement in high-solid anaerobic digestion by adding surfactants was attributed to the interaction between lignocelluloses and surfactants and the extraction of biodegradable fractions from the porous structure. An economic analysis showed that adding Tween 20 was likely to make a profit and be more feasible than adding Tween 60 and polyethylene glycol 300. This study confirms the enhancement in biogas production from horticultural waste by adding non-ionic surfactants.