Process simulation and optimization of municipal solid waste fired power plant with oxygen/carbon dioxide combustion for near zero carbon dioxide emission
Introduction
With the rapid growth of China's economy, the expanding size of cities and the growing population have led to the increasing production of municipal solid waste (MSW). The growing accumulation of MSW has created serious environmental and social problems. The total annual amount of MSW produced in China has reached 191.4 million tons in 2015 [1], and production is predicted to increase to 480 million tons in 2030 [2]. Among the currently used MSW disposal technologies, the waste incineration power generation technology has attracted great attentions due to its advantages of large reducing capacity and energy recovery [3]. However, there exist still some disadvantages in the incineration process due to the complexity of the MSW components [4], such as combustion instability, complex gaseous pollutants and heavy metals emissions. This brings amounts of challenges to the application and the development of incineration technology.
As one of the most promising emission reduction technologies, the oxy-fuel combustion (OFC) technique, which utilizes O2/CO2 mixture as the oxidizer instead of air, can achieve high combustion efficiency, low pollutant emissions and low cost of CO2 capture, etc. [5]. Moreover, the OFC technique can in principle be applied to any type of fuel used for thermal power production [6]. Currently, there have been a few researches related to the MSW O2/CO2 combustion. Through thermogravimetric analysis (TGA) experiment, Tang et al. [7] studied the co-combustion characteristics of plastic, rubber and leather in O2/CO2 atmosphere. They found that the oxygen-enriched combustion technology could alleviate the inhibitory effects of the replacement of N2 by CO2. Also by virtue of non-isothermal TGA, Chen et al. [8] investigated pyrolysis and combustion characteristics of petrochemical wastewater sludge under oxy-fuel condition. The results showed characteristic combustion rates and combustion performance indexes increased while characteristic temperatures decreased with the increase of O2 concentration. Furthermore, Tang et al. [9] focused on the NOx and SOx emission characteristics of MSW combustion in the O2/CO2 atmosphere by the fixed bed combustion experiment. The main influence factors, such as the combustion temperature, the oxygen content, and the Ca/S ratio were analyzed. As expected, NOx and SOx emission reduced under oxy-fuel condition when the temperature was 800–1000 °C. The same conclusion was obtained in another literature [10]. In addition, they also studied heavy metal enrichment characteristics in the ash of MSW combustion in CO2/O2 atmosphere [11]. The results indicated that the replacement of N2 by CO2 increased enrichment of these heavy metals in bottom ash, which was beneficial for their removal. Effects of sorbents on the heavy metals control during tire rubber and polyethylene combustion in O2/CO2 and O2/N2 atmospheres were also studied by Tang et al. [12]. It was found that the replacement of N2 by CO2 can increase heavy metal capture for tire rubber. Based on the publications, one can conclude that the application of oxy-fuel combustion technology to MSW incineration offers an effective solution for overcoming the disadvantages of conventional incineration technology mentioned above, and near-zero CO2 emission may be achieved during MSW disposal.
However, due to the inclusion of the air separation unit (ASU) and the flue gas processing compression system (FPC) in the commercial application, the cycle efficiency of oxy-fuel power plant is greatly reduced by 9–13% compared to the conventional air-firing power plant [13]. This has become the key challenge to its large-scale promotion and application of this oxy-fuel technology. Many studies have been conducted to evaluate the performance of oxy-fuel power plant, and coal and biomass are typically used as the feedstock. Skorek-Osikowska et al. [14] studies a supercritical coal-fired power plant of 460 MW by using commercially available GateCycle and Aspen Plus. Thermodynamic analyses and economic analyses were conducted to evaluate their different structural oxy-fuel systems. It was found that the efficiency decrease of the optimized case was reduced by only 3.5%. In addition, Skorek-Osikowska et al. [15] also founded that the oxygen separation method of hybrid membrane-cryogenic installation can improve the net efficiency of electricity generation by 1.1%. Yan et al. [16] conducted comprehensive sensitivity analyses to assess the performance of a 600 MW oxy-fuel bituminous coal-fired power plant including comparative analyses between dry, half-dry or wet recirculation recycle and concluded that the acid dew point of the wet recycle system is the highest and the dry recycle system is the lowest. By heating the feed water with hot flue gas, the net efficiency of an oxy-fuel process can rise by approximately 1%. Espatolera et al. [17] modeled a circulating fluid bed (CFB) oxy-fuel power plant, including the comparison with oxy-fuel reference power plant and founded that the net electric efficiency of the operational concept power plant is 35.83%, about 3.0% increase compared to the reference power plant. Cormos [18] evaluated the main techno-economic performances of oxy-combustion power plants operated with fossil (coal and lignite) and renewable (sawdust) fuels. Compared to the same supercritical power plant without CO2 capture and storage, the investigated three kinds of fuels oxy-combustion cases showed energy penalties of 9–12% net efficiency, 37–50% increase of total capital investments, and the operational and maintenance costs are increased 7–15%. Most studies have focused on biomass and coal-fired oxy-fuel combustion power plant, whereas very few studies have considered the MSW as a feedstock for the analysis of oxy-fuel power plant. Only Fu et al. [19] evaluated the flue gas heat losses and the economic analysis of MSW oxy-fuel incineration, the results indicated that the scale of the flue gas cleaning system can be reduced due to the reduction of flue gas rate in oxy-enriched incineration, and both equipment investment and operating costs experience a decline accordingly. Moreover, Tang et al. [20] applied the method of Life Cycle Assessment (LCA) to evaluating the entire life cycle of an MSW oxy-fuel incineration power plant. It was concluded that the application of OFC technology in an MSW power plant can reduce CO2 and NOx direct emissions. The electric power consumption of ASU was the primary influencing parameter, and the electric power consumption of CO2 compressor was the secondary.
Based on the published researches on the MSW oxy-fuel combustion power plant systems, several issues are found needing to be further discussed before the MSW O2/CO2 combustion technology can be proper for wide commercial application. First, the analyses of the entire MSW oxy-fuel incineration power plant are still not enough, especially in system or in integrated system coupled optimization to improve its power generation efficiency. Second, the most of reported researches were conceptual, instead of actual MSW power plant, making the results lack of applicability and impossible to be realized. Third, the published literature only studied the proportions of energy consumptions, but the main operation parameters of the integrated system are not widely discussed. Therefore, to get more understanding of the MSW O2/CO2 combustion technology, it is indispensable to put analysis efforts on the entire MSW oxy-fuel incineration power plant.
In this work, an MSW-based oxy-fuel combustion power generation system with near-zero CO2 emission is proposed and simulated with Aspen Plus. An actual conventional waste-to-energy plant (CWEP) operating in Shenzhen, China, firing 800 metric tonnes per day of MSW, is chosen as the base case for comparison. The parameter analysis and the coupled optimization of an MSW O2/CO2 combustion power system are the main work. First, the simulation model of a conventional MSW air incineration plant is constructed, and the simulation results are compared with the operation data to ensure the reliability of the models. Second, the subsystems of ASU, MSW O2/CO2 combustion and FPC are set up and evaluated respectively, beside the sensitivity analysis of combustion processes. Last, the individual subsystems are coupled and integrated to establish the whole process model of a near-zero CO2 emission MSW O2/CO2 combustion power plant. In addition, the coupling optimization and analysis of the process is carried out. The efficiency of the system can be improved after optimization. Results obtained in this work may be utilized for the construction and improvement of MSW oxy-fuel incineration technology.
Section snippets
Process description and simulation
Based on the CWEP, the whole process of MSW O2/CO2 combustion power plant with near-zero CO2 emission is modeled in this work. The commercial software, Aspen plus, serves as the platform for simulation based on the heat and mass balances, which has been widely used in the fields of chemical engineering, coal or biomass combustion and gasification [21]. A process flow diagram for the total system is shown in Fig. 1. The process consists of three primary subsystems: (1) the ASU subsystem; (2) the
Results and discussion
In this section, the simulation results and the sensitivity analyses of the process are discussed after the baseline case is setup and validated. In addition, the coupled optimization and analyses of subsystems as well as the overall MSW O2/CO2 combustion power plant are also discussed.
Conclusions and future remarks
Using the Aspen Plus, each subsystem of the MSW O2/CO2 combustion power plant for near-zero CO2 emission is simulated and analyzed, besides the sensitivity analyses of combustion processes. The whole process of the system is then coupled and optimized. From this work, the following conclusions can be drawn:
- (1)
The power consumption of air compressor is the main part for ASU, and the per unit oxygen consumption is about 0.33 kWh/Nm3. The best supplying oxygen concentration is 96%. In this situation,
Acknowledgments
The authors gratefully acknowledge financial supports from the National Key R&D Program of China (2017YFB0602003), the National Natural Science Foundation of China (NSFC, 51576014 and 51576013) and the Fundamental Research Funds for the Central Universities (2015RC012) for this work.
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