Lignocellulosic feedstocks are year-round, available bio-residues that are the right candidates for counteracting the energy crises and global warming facing the world today. However, lignin leads to a slow hydrolysis rate and is a major bottleneck for biogas production via anaerobic digestion. Anaerobic co-digestion (AcoD) is an economical method available, which overcomes the limitation of a single feedstock’s properties in an anaerobic digestion process. This paper critically reviews the impacts of co-digestion on lignocellulosic biomass degradation, process stability, various working parameters, and microbial activities that improve methane yields. A combination of compatible substrates is chosen to improve the biomethane yield and conversion rate of organic matter. AcoD is a promising method in the delignification of lignocellulosic biomass as an acid pretreatment. Ultimate practices to control the impact of co-digestion on system performances include co-feed selection, in terms of both carbon-to-nitrogen (C/N) and mixing ratios, and other operating conditions. A detailed analysis is performed using data reported in the recent past to assess the sensitivity of influencing parameters on the resultant biogas yield. For the investigators motivated by the basic principles of AcoD technology, this review paper generates baseline data for further research work around co-digestion.

Currently, difficulties such as the depletion of fossil fuel resources and the associated environmental pollution have driven the rise of other energy systems based on green energy sources. In this research, modeling and a viability study of grid-connected and islanded photovoltaic (PV) power systems for supplying the residential load in Mekelle City, Ethiopia, were carried out considering the country’s emerging utility tariff plan for 2021 and beyond. The technical viability of the proposed supply option was analyzed using PVGIS, PVWatts and HOMER Pro tool, while the economic and environmental optimization aspects were carried out using HOMER Pro. Sensitivity analyses and output comparisons among the three renewable energy simulation tools are presented. The results showed that under the consideration of an incremental electricity tariff plan (up to 2021), the analyzed cost of energy of the grid/PV system is around 12% lower than the utility grid tariff. Moreover, we also found that by taking the continuous global solar PV cost reduction into account, the cost of energy of the modeled islanded operation of solar PV power units totally broke the grid tariff in Ethiopia after 2029 based on the tariff for 2021 and well before with the expected escalation of the grid tariff on an annual basis. The technical performance of the system realized through PVGIS and PVWatts was almost comparable to the HOMER Pro outputs. Thus, this investigation will offer a clear direction to the concerned target groups and policy developers in the evolution of PV power supply options throughout the technically viable locations in the country.

This paper presents the kinetic modeling of biomass gasification in bubbling fluidized bed (BFB) gasifiers and optimization methods to maximize gasification products. The kinetic model was developed based on two-phase fluidization theory. In this work, reaction kinetics, hydrodynamic conditions, convective and diffusion effect, and the thermal cracking of tar kinetics were considered in the model. The model was coded in MATLAB and simulated. The result depicted good agreement with experimental work in literature. The sensitivity analysis was carried out and the effect of temperature ranging from 650 to 850 and steam to biomass ratio (S/B) ranging from 0.1 to 2 was investigated. The result showed that an increase in temperature promoted H2 production from 18.73 % to 36.87 %, reduced that of CO from 39.97 % to 34.2 %, and CH4 from 18.01 % to 11.65 %. Furthermore, surface response was constructed from the regression model and the mutual effect of temperature and S/B on gasification products and heating value was investigated. In addition, the desirability function was employed to optimize gasification product and heating value. The maximum gasification product yield was obtained at 827.9 and 0.1 S/B. The response predicted by desirability function at these optimum operational conditions was 30.1 %, 44.1 %, 13.2 %, 12.9 %, 14.035 MJ/Nm3, and 14.5 MJ/Nm3 for H2, CO, CO2, CH4, LHV, and HHV, respectively. Kinetic modeling of the biomass gasification in BFB process is still under development, which considers the diffusion effect, tar cracking, reaction kinetics, and hydrodynamic behavior. Moreover, the large number of previous studies gave priority to a single parameter investigation. However, this investigation can be extended to various parameters analysis simultaneously, which would give solid information on system performance analysis.