Effect of origin and production rate of MSW on the exergoeconomic performance of an integrated plasma gasification combined cycle power plant

Highlights

• As moisture of MSW increases, unit exergy cost increases and efficiency decreases.

• The plasma gasifier and gas turbine have the largest exergy destruction ratio.

• The unit exergoeconomic cost of electricity varied from 11.7 up to 15.6 ¢US$/kWh.

• IPGCC plant feasibility depends on origin, amount, and disposal fee of waste.

• The minimum waste disposal fee for a competitive local plant was 57.6 US$/t.

Abstract

This work studies how different scenarios of the municipal solid waste (MSW) management, moisture content, origin, and production rate affect the exergoeconomic performance of an integrated plasma gasification combined cycle (IPGCC) power plant. Several parameters associated with MSW management are considered as input for the exergy assessment of an IPGCC power plant simulated in Aspen Plus. The feedstock considered is waste from residential (1121 t/day), institutional (75 t/day), commercial (168 t/day), and industrial (104 t/day) sectors, as well as the mixture between them. Thus, for this analysis, several plant scenarios were formulated, according to the amount and moisture content (MC) of the MSW from different origins. Regarding the exergy analysis, the plasma gasifier (PG) and the gas turbine (GT) exhibited the largest exergy destruction ratio, with an average contribution of 36.2% and 40.3%, respectively. Furthermore, as MC increased from 26.6% to 57.9% (according to each waste type), the exergy destruction ratio of the PG increased by 23.5%, leading to a decrease in energy and exergy efficiencies of the IPGCC plant by 23.7% on average. The exergy cost of electricity ranged from 11.7 to 15 ¢US$/kWh. Therefore, the plants with a processing capacity from 100 to 1000 t/day require a MSW treatment fee between 96 and 57.6 US$/t to match the price of hydro-electricity in the Colombian energy market (6.92 ¢US$/kWh).

Introduction

The high generation rate of MSW is an important issue for worldwide societies. According to projections of the World Bank, the global production of MSW will increase up to ~2200 million tons in 2025 [1]. In Colombia, the generation rate of MSW was ~30100 t/day in 2017 [2], of which 83% is disposed in landfills [3]. Landfilling has been associated with problems such as complex and expensive emissions controls, extensive land use, long-time degradation of waste, and low acceptation by population [4]. Furthermore, the MSW sector projections indicate that several material recovery strategies must be implemented in order to reduce landfill disposal, thus avoiding sanitary emergencies by 2030 in the country, due to the short lifespan of active landfills [2], [3].

In this scenario, the energy recovery from MSW for power generation could be an important alternative to face the above-described issues by reducing the amount of waste disposed in landfills [5], [6]. Power production using MSW can be carried out by means of several Waste-to-Energy (WtoE) technologies [5], [7], [8], [9], [10], among which incineration is the most widespread, with an average efficiency of ~20% [11]. Nevertheless, plasma gasification has recently obtained growing attention because of its high efficiency and flexibility [12]. A plasma gasifier could be coupled to several technologies such as internal combustion engines (ICE) with efficiencies of ~30% [13], gas turbines (efficiency of ~30%) [11], [14], or fuel cells (efficiency of ~35–45%) [15], [16]. Therefore, based on the higher efficiency of the Integrated Plasma Gasification Combined Cycle plants (IPGCC) with regard to conventional incineration plants (~30% vs. 20%), as well as its environmental benefits, IPGCC could be an efficient alternative to solve the problems stemmed from the high amount of MSW generated in Colombia [2].

Modeling and simulation are useful tools to develop pre-feasibility studies of power generation by IPGCC plants from the energy, environmental, and economic viewpoints. This allows to obtain significant results with relatively low time and investment costs. Mountouris et al. [17], modeled the plasma gasification of sewage sludge for electricity generation with a gas engine. This system is able to process 250 t/day of sewage slug with a moisture content of 68% to produce heat for drying the feedstock (4.56 MW) and a net electric power of 2.85 MWe, with an efficiency of ~19%. Minutillo et al. [11], and Valmundsson and Janajreh [14] modeled the energy recovery from different waste through IPGCC plants using Aspen Plus, considering temperatures of 4000 °C and 2500 °C for the plasma and gasification reactions, respectively. In the former work [11], the highest efficiency was 31% when using refuse derived fuel (RDF) as feedstock and air as plasma gas. While, the use of waste tires as fuel with steam as plasma gas resulted in an efficiency of 28.5% [14]. Matveev et al., [18] modeled a small scale IPGCC power plant fed with coal, studying the effect of gasifying agent (air, O₂, and steam), gasifier pressure (1.5 – 10 bar), and the gas turbine technology (simple cycle or with regeneration). The highest efficiency (25%−36.4%) was reached with O₂-steam mix as oxidizer, at a pressure level of 7.5–8 bar, and with regeneration in the gas turbine. Another work assessed the co-processing of MSW and plastic solid waste (PSW) in an IPGCC by simulation using the Aspen Plus environment [19]. Mixtures of air with O₂ or steam were considered as plasma gas. The highest efficiency achieved was 38%, when pure O₂ and a mixture of 70% MSW − 30% PSW were fed to the gasifier. These researchers found a direct relationship between the overall plant efficiency and the efficiency of the plasma gasifier. Other works have investigated on the integration of MSW plasma gasification with solid oxide fuel cells for electricity generation, whose efficiency was reported between 27% and 45% [16], [20], [21], [22].

Beyond conventional technical–economic assessment, results from exergoeconomic analysis provide crucial information about the effect of design and operation characteristics of the system, which is not possible to obtain by means of conventional energy and economic analyses [23], [24].

Jack and Oko presented the exergoeconomic analysis of a simulated MSW incineration power plant. The exergy efficiency of this power plant was 31.36%, and the highest exergy destruction rate and cost of exergy destruction were allocated to the incinerator (2.26 MW and 1626.3 US$/h, respectively) due to the large irreversibility associated with the combustion process [25]. The unit cost of electricity was 5.57 ¢US$/kWh, which is higher than that of steam power plants fueled by natural gas. Behzadi et al. [26] combined the thermoeconomic analysis with a multi-objective optimization methodology to investigate and compare autothermal gasification and digestion of MSW for electricity generation in a Rankine cycle power plant. The optimal exergy efficiencies were 17.98% and 19.02% for gasification and digestion-based systems, respectively. The lower performance of the gasifier-based system was a consequence of the moisture content of MSW and the gasification temperature. Kalinci et al. [23] used the Specific Exergy Costing (SPECO) approach to assess a plasma gasification-hydrogen purification system fed with sewage sludge for hydrogen production. The pressure swing adsorption unit was associated with the highest exergy destruction rate (11.66 MW), followed by the plasma gasifier (6.55 MW). Furthermore, the low efficiency of the system (2.6%) and high exergy cost of hydrogen (75.09 ¢US$/kWh) were attributed to the low hydrogen yield in the plasma gasifier (20.23 vol%). Meanwhile, Nakyai et al. [24] analyzed the gasification of biomass with methane co-feeding. The addition of methane has a positive effect on hydrogen production. The highest hydrogen yield was 67.31 mol-H₂/kg_biomass when air (0.21 kg_air/kg_biomass) and steam (1.0 kg_steam/kg_biomass) were used as gasifying agent, and methane was co-fed with a ratio of 0.36 kg_methane/kg_biomass. Consequently, at these conditions, the highest exergy efficiency was 71.8%, leading to the lowest unit exergoeconomic cost of hydrogen (6.8 ¢US$/kWh). Casas Ledón et al. [27] applied the exergoeconomic analysis on a modeled IGCC plant fueled with MSW. The gasifier temperature ranged from 850 to 950 °C to produce syngas with enough energy content to fuel the generation island. The gasifier accounted for nearly 60% of the total exergy losses, and the exergoeconomic cost of the produced electricity ranged from 7 to 13 ¢US$/kWh [27].

Although several previous works have focused on assessing WtoE processes from an exergoeconomic viewpoint, no exergoeconomic assessment studies of IPGCC plants fed with MSW were found in the scientific literature cited. Herein, we present a comprehensive exergoeconomic assessment of an IPGCC plant using MSW as feedstock. For this novel analysis, several plant scenarios were formulated by combining the MC, origin, and production rates of MSW. The aim of this work is to assess the thermodynamic and economic performance of the IPGCC plants for energy recovery from waste, contributing to mitigate the environmental and social issues stemmed from landfill disposal of MSW, as well as to diversify the energy mix.