Thermo-environmental and economic analysis of an integrated municipal waste-to-energy solid oxide fuel cell, gas-, steam-, organic fluid- and absorption refrigeration cycle thermal power plants
Introduction
Energy systems are fundamental and critical to our quality of life, ease of mobility, as well as the growth of our economy. Nigeria electric power demand is estimated as about 25 GW [1] whereas, the peak power generated in the second quarter of 2017 is 4.079 GW [2]. This large unsatisfied electric demand for both domestic and commercial uses calls for urgent and sustainable measures. However, there is a huge waste generation per capita in the urban communities, which is attributed to the high standard of living observed in the cities. Managing Municipal Solid Waste (MSW) in the major cities in Nigeria poses a very big challenge [3]. The hierarchy of waste management is reducing the quantities, re-use and recycle but this has in modern times included the reclamation of energy as preferred to landfilling [4]. Waste generated in Port Harcourt, one of the big cities in Nigeria, has been characterized by Igoni et al. [5] and quantified by Samuel et al. [6], which show the viability of Waste-To-Energy (WTE) application in Port Harcourt. However, advanced thermal treatment technologies, e.g. gasification technology, is required to adequately recover the energy stored in the MSW as well as friendly environmental benefits as demonstrated in Arena [7]. Biomass, e.g. the MSW, is suitable for gasification technology due to the large scale availability of MSW and good physical and chemical characteristics [8].
The composition of syngas obtained from gasification technology depends on the gasification agent, the type and operating parameters of the gasifier [9]. Models of coal and waste gasification have been proposed and simulated in many studies, Perna et al. [10], Areeb et al. [11], [12], and Hassan and Kenan [13] for example. The syngas after reformation to hydrogen has found application in fuel cell technology for power generation, which features high efficiency since this technology is not limited by the limit imposed by the Carnot efficiency [14], [15], [16]. Fuel Cell (FC) system has lower greenhouse gas emissions [17] and exceeds the 40% efficiency target of the Department of Energy (DoE) when compared to other energy conversion technologies [18]. Depending on FC operating temperature and type of fuel utilized, Niakolas et al. [19] classified fuel cell types as: Alkaline Fuel Cell (AFC), Proton Exchange Membrane Fuel Cell (PEMFC), Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC). However, SOFC is flexible to a variety of fuels, namely, H2, CO, CH4 and higher hydrocarbons [20] and is insensitive to fuel contaminants because of its ceramic nature and high operating temperature [9], which favours its adoption in many applications [21]. SOFC systems produce less harmful chemicals and acoustic emissions at high energy efficiency compared to other thermal power production technologies [22].
The energy and exergy efficiencies of combined SOFC and Gas Turbine (GT) is boosted when compared to a stand-alone SOFC or GT as seen in Leyla et al. [23], Haseli et al. [24]; further improvement on system performance is observed when Organic Rankine Cycle (ORC) is integrated such as in Ebrahimi and Moradpoor [25] and Valerie et al. [26]. The performance of SOFC integrated GT cycle and ORC reveals an increase in plant power and efficiency and a decrease in total irreversibility at increasing compression ratio [27], [28], [29], while entropy generation in a hybrid SOFC-GT is minimized at turbine inlet temperature of 1800 K [30]. However, the efficiency could be further increased by utilizing the stack thermal energy as demonstrated by Palomba et al. [31] in an SOFC-CHP coupled with a thermally driven adsorption chiller. Integrated biogas, SOFC and steam injection GT system for electricity generation possess better thermodynamic, economic and environmental characteristics than the simple biogas or SOFC or Steam Turbine (ST) systems as demonstrated in many studies, namely, Mehr et al. [32], Roberto et al. [33] and Nicholas and Shawn [34]. Power generation and efficiency of a gas turbine diminishes when ambient temperature is higher than ISO temperature (15 °C) especially in hot periods of the year. This can be mitigated by cooling the compressor inlet air by conventional technologies such as Absorption Refrigeration (AR), high pressure fogging and evaporative cooling techniques [35], [36]. The application of absorption refrigeration system for compressor inlet air cooling has been seen in many applications since it can be operated by waste heat (e.g. [37]). Detailed energy and exergy analysis on LiBr/H2O pair in AR system has been carried out by Najjar and Al-zoghool [38] and Reynaldo et al. [39] and a Lithium Bromide (LiBr) concentration of 0–70% is recommended to avoid crystallization. In combined power systems, the exergy destruction rates are large in Combustion Chamber (CC), Air Preheaters (AP), exhaust stack and SOFC in various orders of magnitudes as observed in Leyla et al. [23], Memon et al. [40], Ozcan and Dincer [41] and Haseli et al. [24].
Though many research works have been dedicated to SOFC-GT-ORC plants, no paper has adequately considered the energy value chain from MSW to the stack in such a fashion, namely coupling gasification, SOFC, GT, Vapour adsorption, ORC and ST in a single platform. Furthermore, analyses of integrated power plants have been limited to sustainability exponents without due consideration of energo-economic sustainability exponent, which measures how effective the energy conversion processes are and their economic impact on the society in respect to cost and socio-economic conditions. Therefore, the aim of this study is to show the effects of recovering the primary energy in MSW generated in Port Harcourt city by converting same into useful secondary energy in order to ameliorate the energy deficiencies of the city and to combat the MSW related health and environmental hazards. This was achieved by integrating syngas production and power generation through SOFC technology, GT, ST, ORC and AR cycle technologies in a single platform. Therefore, assessing the energy, exergy, environmental and economic performance characteristics of the platform is a novelty in coupling of both high grade and low grade thermal energy systems. Solutions to the models and simulations were realized in the Gasify, EES and MS Excel software environments.
Section snippets
Problem formulation and solution methods
The proposed WTE system is modelled by considering relevant thermodynamic, environmental and economic parameters. First, the system is described using the plant and thermodynamic diagrams in Fig. 1, Fig. 2, respectively. This is immediately followed by the gasification, energy, exergy, economics, environmental and sustainability models of the power-refrigeration system in that order.
Results and discussion
This section discusses the results and the simulations of relevant parameters of the proposed waste driven integrated power plant configuration described in the foregoing section. The input parameters, model validation for gasification and SOFC, MSW energy characteristics, thermodynamic characteristics, economic characteristics, exergoeconomic characteristics and environmental characteristics of the plant are presented in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9
Conclusion
This work, a proposed integrated waste-to-energy power plant, has demonstrated the technical, economic and environmental feasibility and sustainability of using municipal solid waste to generate clean and affordable power for the Port Harcourt metropolis. The analysis of the proposed plant was centered on energy, exergy, economic, environmental and sustainability. The models developed were implemented in the Engineering Equation Solver (EES) and Gasify® while parametric simulations were carried
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