Elsevier

Renewable Energy

Volume 143, December 2019, Pages 64-76
Renewable Energy

Biopower and biofertilizer production from organic municipal solid waste: An exergoenvironmental analysis

https://doi.org/10.1016/j.renene.2019.04.109Get rights and content

Highlights

  • An MSW-fed anaerobic plant coupled with a genset is exergoenvironmentally analyzed.

  • The unitary exegetic environmental impact of biopower is determined at 11.10 mPts/GJ.

  • The highest total environmental impact rate (37.05 mPts/h) is found for the genset.

  • The highest operation-related environmental impact rate is caused by the genset.

  • The digester shows the highest component-related environmental impact rate. .

Abstract

In this study, the environmental performance of a genset-coupled anaerobic digestion plant is analyzed at component-level using an exergoenvironmental method. The plant digests organic municipal solid waste (MSW) while producing two main products, i.e., biopower and biofertilizer. A comprehensive exergoenvironmental modeling of the plant is conducted using actual operating data in order to highlight the main units consuming exergy and causing environmental burdens. The exergoenvironmental indicators of all units of the system are computed by integrating exergy and environmental impact balances. The unitary exegetic environmental impact of biopower and biofertilizer are determined at 11.10 and 0.36 mPts/GJ, respectively. This means that the biofertilizer generation causes less environmental burden over the biopower due to the ease of its production. The highest total environmental impact rate (37.05 mPts/h) is caused by the genset followed far behind by the digester (8.56 mPts/h). Although the genset has the highest operation-related environmental impact rate (36.97 mPts/h), the highest component-related environmental impact rate (7.87 mPts/h) is associated with the digester. Therefore, the exergoenvironmental performance of the plant can be boosted by minimizing the rate of exergy dissipation of the genset while mitigating the environmental impacts related to the development and construction of the digester.

Introduction

The quantity of municipal solid waste (MSW) has increased dramatically worldwide as a consequence of the expanding population, growing urbanization, rapid industrialization, and rising living standards. MSW management is undoubtedly one of the most critical issues of the global environment, particularly in urban areas. If not handled properly, it can seriously affect environmental quality by releasing unpleasant odor, generating leachate, and emitting greenhouse gases. Landfilling is the main disposal route to deal with MSW in the majority of the countries worldwide regardless of the per capita income [1]. However, this treatment method has been criticized because of its high environmental impacts and incompatibility with the concept of circular bioeconomy.

During the past few decades, numerous environmentally-friendly biochemical (anaerobic digestion, fermentation, composting) and thermochemical (gasification, pyrolysis, hydrothermal carbonization) pathways have been developed and commercialized throughout the globe to mitigate the problems associated with MSW disposal. Among the various technologies developed, anaerobic digestion has gained more adherents worldwide because of a high biodegradable organics fraction (40–50%) of MSW collected from households and municipal areas [2]. Using this technically-feasible and economically-viable technology, double goals of waste management and energy recovery can be simultaneously achieved, promoting the circular bioeconomy and sustainable development.

In-vessel anaerobic digestion process as an environmentally sympathetic pathway of MSW treatment produces biogas and nutrient-rich fertilizer under mesophilic conditions at temperatures in the range of 25–40 °C. Biogas consisting of methane, carbon dioxide, hydrogen sulfide, water, and oxygen is evolved by breaking down the organic MSW using a consortium of anaerobic microorganisms under controlled conditions. The obtained renewable fuel can be directly converted into heat, electricity, and mechanical work. In addition, the remained liquid and solid effluents are high-quality fertilizers that can be applied to enhance soil fertility. Since the successful commercialization of anaerobic digestion technology in the late of 1980s [3], various research efforts have been devoted to analyzing organic MSW-fed commercial anaerobic digestion plants aimed at improving their technical, financial, environmental, social, and managerial dimensions [4,5]. Even though an anaerobic digestion process is an attractive option to treat wastes and to provide renewable energy, its sustainability and renewability aspects should still be comprehensively assessed by implementing more elaborate assessment tools like life cycle assessment (LCA) approach and exergetic methods.

LCA is one of the most powerful tools used for examining and measuring the cradle-to-grave environmental consequences of making and using a product/service [6]. This method is started by compiling the relevant inputs and outputs of a given production/servicing system, followed by determining their associated environmental impacts, and is finally finished by translating and interpreting the results achieved in the inventory analysis and impact assessment stages [7]. The multiple impacts on natural resource use, human health, global climate change, and ecological well-being can be reliably measured using this method [8]. Accordingly, LCA approach has been recently applied in several research works to evaluate the environmental impacts of anaerobic digestion plants. For example, Lijó et al. [9] have environmentally analyzed electricity generation from biogas produced through co-digestion of pig slurry, maize silage, and triticale silage by means of LCA methodology. Pérez-Camacho et al. [10] have carried out an LCA analysis to evaluate the environmental impacts of incorporating food wastes into conventional anaerobic digestion feedstocks. Ramírez-Arpide et al. [11] have used LCA method to analyze the environmental impacts of biogas produced from co-digestion of nopal cladodes and dairy cow manure. These studies have shown that LCA is a powerful method for the eco-design of anaerobic digestion systems by determining the quantity of all energy and materials utilized as well as wastes and emissions released into the environment.

On the other hand, derived by consolidating the first and second laws of thermodynamics [12], exergy is the possible work that can be attained from a given thermodynamic system when it is allowed to interact with the surroundings [13]. Unlike conventional energy analysis, exergy analysis can quantitatively and qualitatively value an energy/material flow [14]. In fact, exergy analysis can provide unique insights into more efficient use of energy/materials in waste-to-energy (WTE) systems by identifying, quantifying, and locating the energy quality loss or thermodynamic non-idealities [15]. Accordingly, this engineering method can be reliably employed for measuring the degree of renewability of energy systems by determining the quantity of exergy destruction as resource costs [16]. During the last decade, there has been a great deal of interest in using the exergetic methods for analyzing, optimizing, and retrofitting WTE projects. For instance, Yağlı et al. [17] have exergetically investigated the exhaust waste heat recovery potential of a biogas-fed combined heat and power system through an organic Rankine cycle working at subcritical/supercritical conditions. Prodromidis and Coutelieris [18] have used the exergy concept to investigate a solid oxide fuel cell system fueled with biogas. Mudasar et al. [19] have analyzed an organic Rankine cycle annexation into a sewage plant for waste heat recovery from the exergetic viewpoint. These investigations have revealed the fact that exergy analysis can undoubtedly aid designer and engineers with developing more energy-efficient, cost-effective, and eco-friendly WTE projects.

Given the outcomes of the above investigations, both LCA and exergy methods appear to be increasingly popular engineering tools for investigating renewable energy projects from the sustainability and renewability perspectives. However, these approaches have always been implemented independently in the majority of WTE projects, leading to distinct impacts and/or irreversibility results. In fact, LCA-based decision-making might be misleading as LCA method assesses the projects under investigation regarding environmental constraints while such findings might not necessarily satisfy the thermodynamic goals achieved using the exergy analysis, and vice versa. To this end, exergoenvironmental method interconnecting the LCA constraints with the exergy concepts has been proposed by Meyer et al. [20]. Indeed, exergoenvironmental method benefits from the irreversibility aspect of exergy method and the sustainability feature of LCA approach [16]. Exergoenvironmental analysis can provide a deep understanding of environmental burdens caused by each stream and component of energy conversions systems, thus pinpointing the breakthrough points for further environmental sustainability improvement. Notably, neither exergy analysis nor LCA approach can provide such invaluable information as a stand-alone procedure. In light of that, the so-called exergoenvironmental method has been used in numerous investigations to study and optimize different energy/material conversion systems both thermodynamically and environmentally. For example, the optimum combination of a gas-fueled steam power plant and a site utility system have been exergoenvironmentally sought by Khoshgoftar Manesh et al.. [21]. Aghbashlo et al. [22] have exergoenvironmentally evaluated and optimized biodiesel production from waste cooking oil in an ultrasound-assisted reactor. Aghbashlo et al. [23] have analyzed and optimized acetins synthesis from Amberlyst-catalyzed glycerol esterification process with acetic acid by means of exergoenvironmental approach.

The findings of the above-referred investigations have shown that various WTE systems can be analyzed, planned, and optimized adequately from the sustainability and renewability standpoints via exergoenvironmental analysis. Even though a number of research publications have applied exergoenvironmental approach for investigating different energy conversion systems, there is no report on applying this comprehensive tool for evaluating and scrutinizing a real-world commercial genset-coupled anaerobic digestion plant to the best of our knowledge. Notably, in a typical anaerobic digestion plant, anaerobic microbes are harnessed to assimilate organic wastes within digesters, resulting in the production of biogas and biofertilizer. The resultant biogas can be used to generate renewable biopower using various technologies like fuel cells, gas/steam turbines, gensets, etc. Among these technologies, biogas conversion to biopower using gensets is regarded as a much more practical approach because of its relatively more favorable fuel flexibility and low capital cost. Gensets convert the evolved biogas into mechanical energy, powering an alternator to generate biopower. The thermal energy delivered to the coolant of gensets is also used to provide mesophilic/thermophilic conditions inside digesters.

In our previous studies, exergy analysis [24] and exergoeconomic approach [25] have been used to identify the hotspots of exergy dissipation and cost loss, but without performing a component-level detailed environmental analysis of the plant. Hence, in the present work, the environmental performance of a commercial genset-coupled organic MSW-fed anaerobic digestion plant is conducted using the exergoenvironmental method to highlight the breakthrough points for further explorations. To this end, the real-world operating data and environmental constraints are used in the analysis. Generally, the outputs of such comprehensive analyses can be valuable to researchers and engineers involved in developing and optimizing WTE systems.

Section snippets

Plant definition

A genset-coupled organic MSW-fed anaerobic digestion plant is considered throughout this study. More detailed information on the development and performance of the plant can be found in our previous publications [24,25]. In brief, the plant has been established in Abali in the vicinity of Tehran, Iran, in an area of 30,000 m2. This plant has been designed and constructed by an Italian Company (Seko, Curtarolo, Italy). The required data for both exergy and LCA analyses are obtained from this

Results and discussions

Table 2 provides the thermodynamic properties, exergy rate, unitary exegetic environmental impact, and environmental impact rate of each stream of the plant with respect to their thermodynamic status shown in Fig. 1. It should be noted that detailed and comprehensive insights into the exergetic indicators of each unit of the plant can be found elsewhere [24]. However, in order to facilitate the understanding of the outcomes of the exergoenvironmental method, the results of exergy analysis have

Concluding remarks

In this study, a genset-coupled anaerobic digestion system yielding biopower and biofertilizer from organic MSW is exergoenvironmentally modeled and analyzed on the basis of actual operational data. Specific exergy costing approach is considered in order to determine the exergoenvironmental parameters of all the units of the plant for identifying the hotspots of the environmental impact. The key results and main conclusions of this investigation are as follows:

  • 1)

    The unitary exegetic environmental

Recommendations for further research

Future research works should be conducted to reveal the interactions among the units of the plant by splitting their irreversibility-related environmental impact rates into endogenous/exogenous and avoidable/unavoidable portions using advanced exergoenvironmental analysis. In addition, all the thermodynamic, economic, and environmental aspects of WTE plants should be considered simultaneously for a better understanding of WTE plants thermodynamically, economically, and environmentally. This can

Acknowledgements

The authors would like to acknowledge the support provided by University of Tehran, Biofuel Research Team (BRTeam), and Iranian Biofuel Society (IBS).

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