An innovative waste-to-energy system integrated with a coal-fired power plant

Highlights

• A novel hybrid waste-to-energy system integrated with a coal-fired power plant.

• Several connections are built between the waste-to-energy and coal power production.

• The energy obtained from waste is put into the steam cycle of the coal power plant.

• The waste-to-electricity efficiency is notably promoted with a lower investment.

• Plenty of economic benefits and a much shorter payback period can be achieved.

Abstract

An advanced waste-to-energy system integrated with a coal-fired power plant has been proposed to improve the energy utilization of municipal solid waste. In the new design, the energy gained from the waste-to-energy boiler is employed to heat the feedwater and partial cold reheat steam of the coal power plant, and the feedwater of the waste-to-energy boiler is provided by the heat regeneration system of the coal power plant. Consequently, the energy obtained from the waste incineration products is injected into the steam cycle of the coal power plant, and the waste-to-electricity efficiency can be significantly boosted. Based on a 500 t/day waste-to-energy plant and a 630 MW coal power plant, the proposed hybrid scheme was evaluated compared with the conventional separate one. The results show that the waste-to-electricity efficiency is promoted by 9.16% points with an additional net power output of 3.71 MW, attributed to the suggested integration. Furthermore, the energy-saving mechanism of the novel concept was revealed by energy and exergy analyses. Finally, the new design was economically examined, which indicates that the dynamic payback period of the proposed waste-to-energy plant is only 3.55 years, which is 5.87 years shorter than that of the conventional one.

Introduction

The rapid industrialization and urbanization are boosting the production of municipal solid waste (MSW) worldwide. In China, the annual amount of MSW has exceeded 215 million tons since 2017 [1], which is still swiftly increasing and will probably attain 323 million tons in 2020 [2]. MSW management is a momentous issue, not only in terms of affecting human health but also from the perspectives of environment, society and economics [3]. By the end of 2020, all the MSW should undergo harmless treatment in the large cities of China, and more than 95% of the MSW will be innocuously disposed of in the medium-sized cities, according to the 13th Five-Year Plan of the Chinese government [4]. Therefore, how to deal with MSW in an energy-efficient, environmentally friendly and economically affordable way has drawn particular attention for the past few years in China.

Although plenty of laws/regulations and efforts have been implemented to enhance waste management around the world, further modification on MSW management is still indispensable. As so far, only a limited number of technologies for MSW disposal have been widely applied, and landfilling and incineration are the dominating and preferred approaches in most countries [5]. Landfills are the facilities devised for the safe settlement of MSW with different liners and finally with earth covers [6], however, they clearly have negative impacts on the environment, including the deterioration of landscape, the production of dust and leachate, and the emissions of polluting gases [7]. Hence, there is growing opposition to landfilling because of its disadvantages. Since MSW is actually a resource with huge potential in the recovery of material and energy, waste-to-energy (WtE) can bring the benefits of resource generation and minimization of waste [8]. Incineration is a direct combustion method that converts the feedstock into energy in so-called WtE plants, where the energy production is fulfilled by partially recovering the heat of the combustion products, typically by steam generators [9]. In a WtE plant, saturated steam or hot water is normally generated when there is only thermal demand, whereas, for electricity production or combined heat and power production, superheated steam is necessary [10]. Compared with landfilling, incineration can achieve more than 90% volume reduction and needs no further decomposition, thereby the primary option of waste disposal has gradually changed from landfilling to incineration in China [11]. Waste incineration power generation is contemporarily the prime WtE approach of the country [2]. There are several other alternatives for WtE, such as anaerobic digestion, composting, refuses derived fuel and gasification, but their large-scale applications in engineering are still restricted in consideration of costs and technical maturity. In the grand plan of the 13th Five-Year Period (2016–2020), China has set up ambitious goals for waste management, one of which is that incineration is expected to account for over 50% of the national MSW disposal capacity in 2020 [12].

Nevertheless, the current WtE plants are usually characterized by low efficiency (ranging from 18% to 25% for most cases), especially compared with conventional steam power plants using fossil fuels [13]. The poor performances of WtE plants are mainly induced by the synthetical effects of technical and economic constraints, and the vital points can be identified: a) small scale; b) restrained steam parameters; c) high condensing pressure; d) simple steam cycle (no steam reheating and only few feedwater heaters); e) dramatical waste heat loss; f) overmuch auxiliary power [10]. In consequence, there is enormous room for improving the efficiencies of WtE plants, which is crucial for the sufficient utilization of MSW energy.

A volume of work has been devoted to raising the WtE efficiency, mainly focusing on promoting steam parameters, reducing the energy loss of exhaust gas and integration with other thermal systems. Generally, the live steam parameters of a WtE plant are around 4.0 MPa and 400 °C, which are much lower than those of fossil-fueled power plants [14]. The WtE efficiency will augment with the increases of the live steam parameters, but the heat transfer surfaces are subjected to severe high-temperature corrosion under high steam parameters, attributed to the large concentrations of chlorine, sulphur, etc. in MSW [15]. Thus, increasing the steam parameters of a WtE plant may be strongly associated with new steel/alloy/coating that is capable of functioning at extreme risk of high-temperature corrosion, which is still under developing and relatively costly [16]. Three techniques to achieve higher steam parameters in a WTE plant have been reviewed and examined by Ref. [17], involving dividing the flue gas into two fractions at the grate, reheating partial steam using the saturated steam from the boiler drum and further superheating the steam through the hot exhaust gas of a gas turbine, and these options have been adopted in several WtE plants of Europe. Xu et al. [18] proposed an approach that encapsulates aluminum alloy-based phase change materials in ceramic bricks like traditional refractory bricks in the combustion chamber, by which the superheated steam of over 600 °C could be gained. Assembling radiant superheaters in the sections of a WTE boiler where the flue gas temperatures are very high can contribute to higher steam temperatures as well [19]. Via decreasing excess air [20] and using flue gas recirculation [21], the exhaust gas flow rate can be reduced in a WtE plant and the relative energy loss will decline. Another candidate to diminish the energy loss of flue gas is waste heat utilization, which recovers the energy in the exhaust gas to heat feedwater [22], provide district heat [23], generate power by an organic Rankine cycle [24] and so on. The exhaust gas could even be cooled to below the dew point temperature for recycling the latent heat [25]. Whereas, cooling the exhaust gas is probably confined considering the low-temperature corrosion [26] and the gas cleaning process [27]. A number of studies have been done regarding the incorporation of a WtE system and another thermal system. Consonni [28] and Poma [29] have explored the hybrid design containing a WtE system and a natural gas-fired combined cycle, where the saturated steam produced in a WtE boiler is exported to the heat recovery steam generator of the combined cycle for being superheated, and then fed into the steam turbine serving both the combined cycle and the WtE system. The compositive dual-fuel cycle can reach a much higher electrical efficiency than the separate production, without high-temperature corrosion limitation owing to the external superheating of the heat recovery steam generator. Carneiro et al. [30] developed a comprehensive method to praise a hybrid WtE-gas turbine system, which combines four classic procedures (energy, exergy, economic and environmental analyses) to synergistically inspect the availability of such a system. The possibility of converting waste into syngas by gasification and being burned in internal combustion engines for power generation has been investigated by scholars [31, 32]. Also, the co-combustion of MSW and coal/biomass has been studied, but which is still difficult to be applied in practice [33, 34]. Ismail et al. [35] explored the hydrothermal decomposition process that converts the organic constituent of waste into solid fuel, which can be commercially utilized for co-firing with coal in coal-fired power plants. In addition, Mendecka et al. [36] introduced an integrated solar-WtE system that superheats the saturated steam of the WtE boiler in an external heat exchanger receiving heat from the solar tower. Above all, much research has been performed on the advancement of WtE, while little literature has been published concerning the integration of waste incineration power generation and coal-fired power generation based on steam cycles. If a WtE system can be organically incorporated with a coal-fired power plant, the useful energy acquired from MSW incineration may be utilized in a more efficient way and there may be huge potential in the performance improvement of WtE.

Coal provides about 40% of the world’s electricity, more than any other sources [37]. Coal-fired power plants (CFPP) possess the advantages of reliability, affordability, abundance, known technologies, safety and efficiency [38]. Coal power dominates the electric supply of China, where the installed capacity of CFPPs surpassed 60% of the total generation capacity and CFPPs satisfied over 64% of the national electric demand in 2017 [39]. It is predicted that coal will still contribute to beyond 50% of the country’s power supply in 2030 [40]. According to the power statistics of China (2017), the average net efficiency of CFPPs attains 39.81%, which is well above that of WtE plants [39]. On condition that a WtE system can be organically integrated with a CFPP, there may be immense potential in enhancing the overall efficiency.

Against this backdrop, a novel WtE system merging with a large-scale CFPP on the basis of multiple couplings has been put forward. In the proposed design, integration is chiefly accomplished by using the saturated steam of the WtE boiler to warm the feedwater of the CFPP and heating the partial cold reheat steam of the CFPP in the superheater (SH) of the WtE boiler. Besides, the feedwater of the WtE boiler is taken from the heat regeneration system of the CFPP. As consequences, more electricity will be converted from MSW and the waste-to-electricity efficiency can be significantly improved. Moreover, several prime components of the WtE plant (turbine, generator, stack, etc.) can be saved in the new configuration, which reduces the investment of the WtE system. The performance of the hybrid scheme was assessed in the views of thermodynamics and economics, based on a 500 t/day WtE plant and a 630 MW CFPP. The benefits of the option of adopting the novel concept were determined as compared to the conventional separate option, under a power-boosting mode. The root cause of energy conservation due to the innovative design was revealed via energy and exergy analyses. This work may contribute to facilitating the energy utilization of MSW and providing a science and technology foundation for advancing the WtE technology.