Anaerobic digestion as a sustainable technology for efficiently utilizing biomass in the context of carbon neutrality and circular economy

Carbon emissions and associated global warming have become a threat to the world, the major contributor being the extensive use of fossil fuels and uncontrolled generation of solid wastes. Energy generation from renewable energy sources is considered an alternative to achieving carbon neutrality. Anaerobic digestion (AD) is a sustainable technology that has been endorsed as a low-carbon technology complimenting both waste management and renewable energy sectors. The AD technology recovers the volatile matter from waste biomass as much as possible to produce biogas, thus reducing carbon emission as compared to open dumping or burning. However, there is a need of compilation of information on how each subsystem in AD contributes to the overall carbon neutrality of the entire system and chances of achieving a circular economy along with it. Therefore, this article aims to clarify the associated internal and external factors that determine the low carbon characteristic of anaerobic digestion technology. From this review, the potential of AD system for energy-atmosphere-agriculture nexus has been explored. Carbon emission mapping of the potential entities involved in AD were identified and perspective to life cycle assessment and future research direction has been pointed out. Climate change impact and acidification potential are the two entities that can influence the overall environmental sustainability of an AD system. It was recognized that each stage of AD system starting from substrate supply chain, biogas production, upgradation, utilization, and digestate application had a remarkable effect on the overall carbon emission potential based on its design, operation, and maintenance. Selection of suitable substrates and co-digesting them together for improved biogas production rate with high methane content and proper digestate post-processing and storage can vastly reduce the carbon emission potential of the AD technology. Further, a case scenario of India was assessed considering the utilization of major surplus biomass available through AD. Re-routing the three major substrates such as agricultural crop residues, animal wastes and organic fraction of municipal solid wastes through AD can reduce at least 3.5-3.8 kg CO2-eq per capita of annual carbon emission load in India. Furthermore, the pathways in which the policy and legislations over establishment of AD technology and how to explore linkages between achieving circular economy and low carbon economy for Indian scenario has been highlighted.

An improved numerical model to predict the operating temperature and efficiency of solar photovoltaic systems

Solar photovoltaic (PV) technology has a huge potential for producing renewable energy and reducing greenhouse gas emissions. An increase in the PV cell temperature in real operating conditions reduces the actual output of a solar PV system. A 1D transient multi-layered model, based on the fundamentals of the finite difference method, has been developed to predict the operating cell temperature. Since a PV system operates in stochastic wind conditions and is not subjected to any predefined thermal boundary condition, several expressions of convection coefficient have been scientifically analyzed to determine the most suitable expression. The novel calculation approach assumes explicit radiation terms and implicit convection terms to linearize the equations and get rid of any iterative process. Comparison with experimental results shows that the convection coefficient derived from boundary layer theory corresponding to uniform heat flux predicts the cell temperature with the best accuracy showing a mean error of only [Formula: see text] and [Formula: see text]. Splitting the heat source across different solar PV layers produces a maximum change of [Formula: see text] only and can be avoided due to the involved complexity. The study proposes a new piece-wise function for PV efficiency in terms of cell temperature and irradiation. This novel function predicts PV efficiency on a sunny and a cloudy day with [Formula: see text] and [Formula: see text] mean errors, respectively, which are considerably lower than errors obtained using other popular functions in the literature. The model helps in predicting actual output from a PV system more accurately which should enable taking more informed decisions regarding the location of installation, PV technology, and the need for a cooling method.

Stabilization of anaerobic digestion of kitchen wastes using protein-rich additives: Study of process performance, kinetic modelling and energy balance

Anaerobic digestion (AD) of acidic kitchen waste (KW) streams is found to be unstable and leads to poor overall efficiency. This study assessed the effect of pongamia de-oiled cake addition on KW-AD. High acidic KW (pH: 2.00-5.00), medium acidic KW (pH: 5.00-7.00) and low alkaline KW (pH: 7.00-8.00) fed into digesters I, II and III at 10% total solids (TS) achieved biogas yields of 177.82 ± 19.30, 216.57 ± 7.42 and 280.45 ± 2.55 L/kg VS. d, respectively. Maximum synergistic effect of pongamia de-oiled cake was observed in digester I with increased methane production of 46.04% and volatile solids reduction of 11.18%. The principal component analysis and kinetic evaluation revealed that pongamia de-oiled cake addition had a positive effect on the AD parameters in all digesters. With energy efficiencies exceeded 96% in all the digesters, the study proposes the addition of protein-rich additives for KW-AD stabilization.

Characterization of leaf waste based biochar for cost effective hydrogen sulphide removal from biogas

Installation of decentralized units for biogas production along with indigenous upgradation systems can be an effective approach to meet growing energy demands of the rural population. Therefore, readily available leaf waste was used to prepare biochar at different temperatures and employed for H₂S removal from biogas produced via anaerobic digestion plant. It is found that biochar prepared via carbonization of leaf waste at 400 °C effectively removes 84.2% H₂S (from 1254 ppm to 201 ppm) from raw biogas for 25 min in a continuous adsorption tower. Subsequently, leaf waste biochar compositional, textural and morphological properties before and after H₂S adsorption have been analyzed using proximate analysis, CHNS, BET surface area, FTIR, XRD, and SEM-EDX. It is found that BET surface area, pore size, and textural properties of leaf waste biochar plays a crucial role in H₂S removal from the biogas.