• Energy Research

Abstract The anaerobic co-digestion (AcoD) of organic substrates has reportedly shown synergistic effects, but the current indices used to evaluate the magnitude of such effects cannot well support the findings and in some cases bring about confusion. In this paper, meta-analysis was performed through a database compiled from 124 peer-reviewed articles focused on the AcoD of livestock manure. Six commonly used indices for evaluating synergistic effects were statistically analyzed in order to obtain a representative evaluation index. Moreover, the synergy intervals of substrate mixing were preliminarily analyzed on element and molecular levels using the optimized index. The results indicated that the ratio of experimental values to weighted values is a relatively optimized evaluation index. Methane production and organic degradation were found to be two independent indices, thus both of them should be considered simultaneously when AcoD process is under consideration. From a statistical standpoint, the recommended synergy interval of carbon-nitrogen ratio is 20–27. A higher probability of synergy can be achieved when the lipid/carbohydrate ratio is higher than 0.13, as well as the protein/carbohydrate ratio is greater than 0.26. Moreover, a relatively complete execution strategy of AcoD has been preliminarily proposed in terms of selection of co-substrate, mixing ratio, and synergy evaluation. The results achieved herein would have some biases and uncertainties due to the fact that only few studies explicitly demonstrated carbohydrate forms, especially lignin. Therefore, the evaluated indices should be further verified and improved by including more details about carbohydrate forms.

Abstract The greenhouse areas in Iran have expanded rapidly and the greenhouse holders have shown a great tendency to cultivation of those crops that used to be cultivated in open fields. Although, greenhouses are intensive in terms of yield and whole year production, they are considered being one of the major contributors to greenhouse gases (GHG) emissions in the agricultural sector. In the present study strawberry cultivation in greenhouses (GH) and open fields (OF) was selected as a representative of those crops which can be grown in both systems. Initial data were randomly collected from 70 OFs and 33 GHs in province of Gilan, Iran. Energy consumption and GHG emission of two different strawberry production systems were compared. Moreover, energy use efficiency of GH producers due to more energy consumption was studied, then degrees of technical efficiency (TE), pure technical efficiency (PTE) and scale efficiency (SE) were determined using data envelopment analysis (DEA). Additionally, the amount of energy inputs wasted in inefficient greenhouses was assessed and energy saving was computed. Furthermore, the effect of energy optimization on GHG emission was investigated and the total amount of GHG emission was calculated. The total average of energy input and output was estimated at 35,092.4 MJ ha−1 and 10,405.9 MJ ha−1 for OF production and, similarly, 1,356,932.8 MJ ha−1 and 137,772.4 MJ ha−1 for GH strawberry production. Total GHG emission was calculated as 803.4 kg CO2eq ha−1 and 35083.5 kg CO2eq ha−1 for OF and GH production, respectively. Based on the evaluations 20.2% (273,902.8 MJ ha−1) of overall energy sources can be saved if the performance of inefficient farmers is enhanced. Optimizing energy in the greenhouse production can result in a significant reduction in total GHG emission and the present emission of GHG can be reduced to the value of 29309.1 kg CO2eq ha−1.

Three distinctive start-up strategies of biogas reactors fed with source-sorted organic fraction of municipal solid waste were investigated to reveal the most reliable procedure for rapid process stabilization. Moreover, the experimental results were compared with mathematical modeling outputs. The initial inoculations to start-up the reactors were 10, 50 and 100% of the final working volume. While a constant feeding rate of 7.8gVS/d was considered for the control reactor, the organic loading rate for fed-batch reactors with 10 and 50% inoculation was progressively increased during a period of 60 and 13days, respectively. The results clearly demonstrated that an exponentially feeding strategy, considering 50% inoculation relative to final volume, can significantly decrease the alternatively prolonged period to reach steady conditions, as observed by high biogas and methane production rates. The combination of both experimental and modelling/simulation succeeded in optimizing the start-up process for anaerobic digestion of biopulp under mesophilic conditions.

Abstract An ever increasing demand for animal protein products has posed serious challenges for managing the increasing quantities of livestock manure. The choice of treatment technologies is still a complicated task and considerable debates over this issue still continue. To build a clearer picture of manure treatment framework, this study was conducted to review the most frequently employed manure management technologies from their state of the art, challenges, sustainability, environmental regulations and incentives, and improvement strategies perspectives. The results showed that most treatment technologies have focused on the solid fraction of manure while the liquid fraction still remains a potential environmental threat. Compared to other waste to energy solutions, anaerobic digestion is the most mature technology to upgrade manure's organic matter into renewable energy, however the problems associated with high investment costs, operating parameters, manure collection, and digestate management have hindered its developments in rural areas in developing countries. Bio-oil production through hydrothermal liquification is also a promising solution, as it can directly convert the wet manure into biofuel. However, lipid-poor nature of manure, operational difficulties, and the need for downstream process to remove nitrogenous compounds from the final product necessitate further research. Livestock manure management (both solid and liquid fractions) under biorefinery approach seems an inevitable solution for future sustainable development to meet circular bioeconomy requirements. Much research is still required to establish a systematic framework based on regional requirements to develop an integrated manure nutrient recycling and manure management planning with minimum environmental risks and maximum profit.

Abstract Gaseous and liquid anaerobic digestion (AD) streams, currently are at best used for electricity and heat production or simply spreading at the fields, respectively. However, electricity and heat are economically produced from other renewables and advanced fertilizers are needed to avoid leaching and boost nutrients capture. Hence, AD seeks new opportunities to support circular bioeconomy. The overall objective of this review is to present state-of-the-art resource recovery routes for upcycling the AD streams to reduce carbon footprint and formulate alternative products to increase sustainability. Technical barriers and integrated systems to upcycle AD streams through biological means are presented. New technologies and methods to capture CH4, CO2 and nutrients from the digested residual resources are presented, as a) methanotrophs cultivation to be used as feed ingredients; b) CO2 conversion and micro-nutrients capturing from microalgae to be valorized for a wide range of applications (e.g. biofuels, food and feed, fertilizers, bioactive compounds); c) CO2 transformation to biodegradable plastics precursors (e.g. Polybutylene succinate, Polyhydroxyalkanoate); d) digestate valorization for biochar production to support efficient agricultural usage. Moreover, the environmental factors and life cycle assessment perspectives of the novel biorefinery routes are revised highlighting the need for regionalized models or assessments that can reveal the most sustainable routes based on local conditions and requirements. Despite AD poses some positive characteristics related to environmental benefit and emissions reduction, the present work reveals that the novel routes can further enhance sustainability metrics supporting circular bioeconomy.

Anaerobic digestion of chicken manure was carried out in this study basing on central composite design to identify the most optimal strategy for biochar supplementation. Model of cumulative methane production (CMP) was established by using response surface methodology. The optimal conditions predicted accordingly, including manure loading of 51.8 g VS/L, biochar dosage of 3.3% VSmanure, and cellulose loading of 98.0 g VS/L, were expected to maximize CMP, i.e., 294 mL/g VSmanure. The results also demonstrated that biochar dosage and its interaction with manure loading were key factors with significant impact on CMP. Biochar dosage higher than 3.5% VSmanure was observed to weaken the transformation of organic substances to methane. Higher dosage of biochar could considerably reduce concentration of organic acids, total ammonia nitrogen, as well as soluble salts. Verification experiment supported validity of the optimal strategy and provided data for cost assessment, which showed positive cost balances from biochar supplementation.

Abstract The self-heating of lignocellulosic biomass, when degraded under anaerobic conditions, has been reportedly observed. Such a phenomenon would cause substrate to show exothermic characteristics which are in contrast to the current thermodynamic or microbiological knowledge. Rice straw (RS), when undergone anaerobic digestion, is prone to self-heating which can increase the reactor temperature according to heat-releasing characteristics, thus improve the net energy production. In the present study, the relationship of biogas production and self-heating release as well as the synergistic effects of two energy production pathways was investigated. Moreover, the optimal process for maximizing the biogas production under self-heating phenomenon was scrutinized. Compared to control assays, a 0.48 °C increase in average temperature was noticed among experimental trials due to RS self-heating phenomenon. The results showed that the self-heating was even improved simultaneous with increased methane production rate. Hydrolysis rate and total solid content were found to be possible promising options to control the self-heating release.

Liquid transportation biofuel production is a promising strategy to reduce greenhouse gas emissions. Hydrothermal gasification (HTG) has shown great potential as an effective method for valorizing wet biomass. The high-quality syngas produced using the HTG process can be chemically/biochemically converted to liquid biofuels. Therefore, this paper aims to comprehensively review and critically discuss syngas production from biomass using the HTG process and its conversion into liquid biofuels. The basics and mechanisms of biomass HTG processing are first detailed to provide a comprehensive and deep understanding of the process. Second, the effects of the main operating parameters on the performance of the HTG process are numerically analyzed and mechanistically discussed. The syngas cleaning/conditioning and Fischer-Tropsch (FT) synthesis are then detailed, aiming to produce liquid biofuels. The economic performance and environmental impacts of liquid biofuels using the HTG-FT route are evaluated. Finally, the challenges and prospects for future development in this field are presented. Overall, the maximum total gas yield in the HTG process is obtained at temperature, pressure, and residence time in the range of 450–500 °C, 28–30 MPa, and 30–60 min, respectively. The highest C5+ liquid hydrocarbon selectivity in the FT process is achieved at temperatures between 200 and 240 °C. Generally, effective conversion of biomass to syngas using the HTG process and its successful upgrading using the FT process can offer a viable route for producing liquid biofuels. Future studies should use HTG technology in the biorefinery context to maximize biomass valorization and minimize waste generation.