1. Introduction
Considerable reduction in fossil fuel resources, greenhouse gases (carbon dioxide, Sulfur dioxide, and nitrogen oxide) emission, and environmental issues, such as global warming and acid rain, prompted the researchers to explore carbon-free and more environmentally benign energy sources [
1]. Renewable energy sources, such as biofuels, are receiving significant attention worldwide. Being available extensively and replenishable, biomass and wastes from various resources such as residues of agricultural, forestry, dairy, and domestic activities can be regarded as sustainable resources [
2]. Moreover, biomass and various wastes can be employed to drive several systems for such products as electricity, heat, cooling, potable water, ethanol, and renewable biodiesel, through various conversion pathways [
3].
Among the conversion pathways, biomass gasification is an efficient method for syngas production. Gasification of biomass and wastes includes partial oxidation in a closed chamber that yields to gaseous products [
4]. Among the different gasifier types, the fixed-bed downdraft gasifier is recognized as advantageous since it has a relatively high conversion efficiency compared to other gasification methods [
5].
The final syngas produced during the gasification process is usually used in subsequent conversion processes, including combustion in a gas turbine. The concept of integration of the gasification process with a gas turbine cycle for power generation was introduced by Weil [
6] in 1950. The integrated gasification combined cycle (IGCC) combines a solid fuel gasification unit with an integrated gas turbine–steam power plant to enhance performance and achieve higher efficiency [
7]. To enhance the power generation capacity, the off-gases of the gas turbine are used as an energy source to drive a steam cycle through a heat recovery steam generator (HRSG) [
8]. For instance, Soltani et al. [
9] analyzed an integrated biomass gasification and gas turbine combined cycle from energy and exergy perspectives. They showed that, for a particular gas turbine inlet temperature and cold-end temperature difference of the heat exchanger, the thermal efficiency of the system could be maximized by adjusting the pressure ratio. Gholamian et al. [
10] proposed a novel multi-generation system based on an IGCC, which includes a biomass gasification system, a gas turbine cycle, a supercritical CO
2 cycle, and a domestic water heater. For the case of wood as the biomass input, the highest exergy efficiency was calculated to be
. Ahmadi et al. [
11] applied a steam Rankine cycle (SRC) to recover the waste energy of a micro gas turbine, and analyzed the system from thermodynamic, environmental, and exergoeconomic viewpoints. They also carried out optimization, and employed evolutionary algorithms to determine the optimum design parameters. Köse et al. [
12] investigated the utilization of a steam Rankine cycle and an organic Rankine cycle (ORC) as the bottoming cycle of the gas turbine. They performed a parametric optimization to analyze the effects of various working fluids on the ORC subsystem. Optimum performance criteria were obtained for the overall system, including the SRC-ORC bottoming cycle with R141b as the working fluid. Optimal values of energy efficiency, exergy efficiency, and the net output power were evaluated as
,
and
, respectively.
Freshwater shortages are significant concerns for many nations, especially in the Middle East. Water desalination processes are options for addressing such problems [
13]. The main technologies of water desalination are thermal and membrane methods. Thermal desalination processes include multi-effect thermal vapor compression (ME-TVC), multi-effect distillation (MED), and multi-stage flash (MSF), while membrane processes mainly include reverse osmosis (RO) and electrodialysis (ED) [
14]. Thermal desalination processes, especially those combined with power generation cycles, constitute a considerable share of the global desalination market; in the Persian Gulf region, for example, thermal processes account for
of the desalination industry [
15]. Among the different types of thermal desalination systems, MED units have several advantages: including (1) exploitation of low-grade energy sources such as solar energy, geothermal energy, and waste energy [
16], and (2) straightforward integration of MED units into energy systems [
17]. Hence, a promising application for heat recovery is with thermal desalination systems, especially MED units. Baccioli et al. [
18] employed a MED system for waste heat recovery in an ORC plant, and showed that the hybridization of the MED unit to ORC decreases the payback period and lessens the initial cost. An integrated plant comprising GT, MED, and RO systems for electricity and freshwater production was developed by Mokhtari et al. [
19]. They concluded that the total cost could be decreased from
to
by supplying excess power to the RO system.
In addition, several studies have been performed to evaluate the achievements and challenges in the establishment of eco-efficient water infrastructure towards sustainable urban development. Chini et al. [
20] examined the relationship between water and energy in typical household appliances and fixtures. They revealed that applying a cost abatement analysis for the average U.S. household yields a potential annual savings of 7600 kWh and 39,600 gallons per household. Notaro et al. [
21] presented a decision support tool to analyze the water and energy balance in the integrated water service in the Favara di Burgio system (Sicily, Italy). They revealed that the decision support tool could offer efficient solutions, according to the operator objectives, concerning energy and water loss management. Additionally, the proposed method could provide guidelines for choosing the best management solutions, depending on the particular analyzed system, and allow, at the same time, energy and water resources saving. Freni and Sambito [
22] studied the principal energy saving and recovery measures that can be employed in complex integrated urban water systems. By using such techniques, the reduction of water losses can be obtained through the control strategies, resulting in decreased energy consumption and environmental impact.
As in traditional power systems, IGCC systems include various components, where the failure of any part can lead to the failure of the entire system. Hence, the study of the failure and the reasons for the failure offer better identification of the system and precise determination of the final cost of the products [
23]. Few researchers have analyzed the availability and reliability of cogeneration systems. Two component-reliability-importance metrics, including FCI (failure cost importance) and PI (potential failure cost importance), were developed by Jiang et al. [
24] for assessment of the maintenance prioritization of constituents in a combined cooling, heating, and power cycle. A combined system consisting of a multi-stage flash (MSF) desalination unit, a gas turbine (GT), and an HRSG unit was studied by Hosseini et al. [
25] from thermodynamic and reliability viewpoints. They determined that the application of reliability analysis could increase the water and electricity generation costs by about
and
, respectively. A reliability and economic study of a hybrid system of gas turbine cycle and an MSF desalination unit was performed by Arani et al. [
26]. They applied the variable reliability over the operating lifetime, and observed that the payback period increased about nine months, while the net present value (NPV) decreased by around USD 18 million. Wang et al. [
27] developed a modified exergoeconomic analysis considering space-state reliability assessment to study the cost assignations in a biomass-based multi-generation plant. Their results revealed that the gasification system’s repair and failure rates considerably affect the product cost. In addition, by considering reliability analysis, an approximate rise of
in the specific exergy cost of the products can be observed. Razmi and Janbaz [
28] performed an exergoeconomic analysis considering reliability and availability indices on an electricity and distilled water cogeneration system. Their results determined that the cost of electricity and distilled water was improved by
and
, by accounting for reliability considerations.
Main Novelties and Contributions
As discussed in previous paragraphs, based on the benefits of integrated gasification combined cycles (IGCCs) and multi-effect distillation (MED), a cogeneration plant for providing electricity and freshwater is proposed. The devised system’s main novelties lie in the integration of biomass gasification and a regenerative gas turbine with intercooling and a syngas combustor, where the syngas produced in the gasifier is burned in the combustion chamber and directly fed to a gas turbine. In most previously published papers, the combusted syngas is used indirectly through heat recovery systems for various commodity production aims. Using syngas produced in the gasifier directly may lead to various challenges, such as tar and char production through the gasification process. To address these problems, the gasification process is properly designed using a fixed-bed downdraft gasifier and gas cleaning unit. The downdraft gasifier is categorized among the cocurrent reactors where the produced syngas flows down through the high-temperature hot ash bed, which enables the gasifier to favorably crack the tar particles. The syngas produced leaves and ash drops to the bottom of the gasifier. For this reason, the lowest tar content () is produced in a downdraft gasifier among various types of gasifiers. In addition, the fixed-bed downdraft gasifier matches more appropriately the combustion chamber since a shorter time is needed to fire and bring the plant to the operating mode in a downdraft gasifier in comparison to other types of gasifiers. Furthermore, using a proper gas cleaning unit makes the produced syngas applicable in combustion chambers.
Waste heat produced in the topping plant is a favorable option to supply the energy needed for desalination units. Low-grade waste heat has been defined as the heat that is not economically efficient to be recovered in the relevant process. Waste heat utilization is a viable choice to raise energy efficiency, which can reduce global carbon-dioxide emissions. Thus, the energy discharged from the gas turbine is utilized through a HRSG for further electricity and freshwater generation. Another key motivation of the proposed system is the combination of a SRC and a MED desalination unit, where the latter device acts as the condenser in the SRC plant. Such an arrangement results in higher thermal and exergy efficiencies than either plant would have individually.
In addition, despite various studies on IGCC-based cogeneration systems from multiple viewpoints, the availability and reliability of IGCC-based cogeneration plants have not been studied extensively. Hence, the proposed electricity and freshwater production plant is evaluated from the reliability and availability points of view by employing the Markov theory. A multi-objective optimization is carried out to determine the optimum values of the main design parameters and objective functions using an artificial neural network (ANN), with MOPSO as the optimization algorithm and the TOPSIS method as the decision-maker.