Thermodynamic analysis of 350 MWe coal power plant based on calcium looping gasification with combined cycle

https://doi.org/10.1016/j.ijggc.2021.103439Get rights and content

Highlights

  • A novel calcium looping gasification (CLG) in situ CO2 capture in dual fluidized bed is proposed for combined cycle.

  • CLG, CaO-CaCO3 strategy is applied, resulting 87.9% carbon capture efficiency.

  • Efficiency of CLG with combined cycle significantly increased by 2% as compared to IGCC without CCS.

  • Energy and exergy efficiencies of CLG with combined cycle were calculated as 55.0% and 51.7% respectively.

Abstract

Carbon capture and sequestration (CCS) technology is needed for fossil fuels (FFs) based power generation, in order to make it sustainable in future. Calcium looping gasification (CLG) is a novel energy technology that fulfils CCS, unlike IGCC post combustion, CO2 capture is not required, making it less energy demanding and saving capital cost, moreover CaO-CaCO3 is an exothermic reaction, hence the generated heat is adequate to perform CLG. In this study, both experiments and simulations of CLGCC, with CaO as a CO2 sorbent, were performed at varying gasifier temperatures and pressures using Aspen Plus, while results were compared to maximize the overall efficiency. Here, we propose calcium looping gasification with combined cycle (CLGCC) simulation using Aspen Plus with CaO as CO2 sorbent, and results are validated against experimental data in terms of gasifier temperature and pressure. The IGCC without CCS system along with CLGCC are simulated simultaneously for comparison. The CLGCC system achieved an efficiency as high as 54.93%, while IGCC's system without CCS efficiency was calculated as 53.22% and corresponding exergetic efficiencies are 51.67% and 50.03%, respectively. The carbon capture efficiency of CLGCC is 83.5 % with 94.4 vol% of CO2 achieved after steam condensation for carbon capture system. On the basis of results, we demonstrate the applicability of CLGCC against the conventional power plants and it has a great window for adaptation in the future in advanced power plants.

Introduction

The energy consumption in 2019 was about 76 GJ/person/year, which is a 0.2% increase from 2018, and this demand is growing with the growth in population and advancement in modern technology (British Petroleum, 2019). Currently, about 85% of overall energy is produced from FFs (34% oil, 24% natural gas and 27% coal)(Abdalla et al., 2018; British Petroleum, 2019; Shrestha et al., 2016), which release massive amounts of CO2, causing global warming, economic problems and political issues (Abdalla et al., 2018). The 2030 forecast shows that coal will likely remain the primary fuel for power generation in thermal power plants as it is abundant, price competitive and has a wide distribution network on the surface of earth (Smoliński, 2011). By 2040, it is forecasted that it will be used in 23% of the total power generation (Breeze, 2019). Even with the development of renewable energy resources like solar power, wind turbines, biomass and waste utilization, coal use is expected to continue for power generation for a few decades more. With the development of carbon capture and sequestration (CCS), coal-fueled power generation will be acceptable (Fan, 2010; Yan et al., 2013).

Coal is utilized for power generation by direct combustion and gasification, resulting in CO2 emission. To avoid as much CO2 emission as possible, researchers are working to develop different techniques. Initially coal was fired in power plants, and by combustion of coal, high-pressure steam was generated which was used to drive the steam turbine-generator. The traditional combustion power plant has efficiency of one-third of energy input. Secondly, combustion gases are N2 rich which means CO2 concentration is very low and it makes CCS inefficient and expensive (Fan et al., 2008). Further R&D efforts can increase the efficiency of coal combustion power plants up-to 45–55% by super and ultra-supercritical pulverized coal (USPC) technology (Breeze, 2019; Fout et al., 2015).

Another approach was made through coal gasification. Where coal was partially oxidized by air, oxygen, or steam to generate syngas. The syngas mostly consists of CO, CO2, H2 and CH4 along with pollutants such as H2S, ammonia and mercury (Higman and van der Burgt, 2003). By eliminating these impurities, the syngas is sent to a combined cycle (“gas turbine - steam turbine” system) for generation of electricity which is a worldwide technology, known as Integrated gasification combined cycle (IGCC) process. IGCC, presently, stands at 53% efficiency without CO2 capture. While considering CO2 capture, the efficiency decreases sharply up to 43–45%, which can be increased to 47–50% by sequential advances in the gas turbine (GT), gasification and gas cleaning technologies (Promes et al., 2015). Even, the best IGCC configuration with CCS, mostly results in vigorous energy penalties and substantial costs. The cost of electricity was 60% higher than the cost of electricity from traditional coal combustion plants. In the long run, it is essential to evaluate potential methods for power generation from coal with intrinsic CO2 capture. To increase the efficiency, lower down the capital cost, and avoiding the complexity and achieving ease in CCS for the IGCC, the calcium looping concept in coal gasification was introduced, which showed promising results.

Calcium looping is not a new concept, as it was first introduced in the mid of twentieth century. In the beginning, it was applied to combustion processes for electricity generation from gaseous fuels. Since the last two to three decades, calcium looping gasification (CLG) is being applied to produce hydrogen from coal and coal derived syngas. In this process, CaO, as a sorbent, is applied in-situ with capturing CO2 during gasification. Fig. 1, shows the basic concept of CLG. A gasifier reactor and a combustor reactor are two main constituents which are interconnected. Coal is gasified in the gasifier with steam, producing CO2, which is then captured by carbonation of CaO and transferred from gasifier to combustor, where unreacted char is combusted with air or oxygen to decompose the formed CaCO3 into CaO, which is then recirculated to gasifier. During all the process of CO2 capture, the enhanced hydrogen production reactions, like water-gas, water-gas-shift and steam methane reforming reaction take place, according to Le Chatelier's Principle. After all this process, H2-rich syngas and CO2 rich flue gas is obtained from gasifier and combustor reactor respectively. The important reactions involved in CLG are summarized in Table 1.

A great number of novel calcium looping cycle incorporating coal gasification processes are proposed for CCS. They include the Zero Emission Coal Technology (ZEC) and HyPr-Ring process (Lin et al., 2002). Numerous studies have been conducted regarding steam gasification in dual fluidized bed (DFB) for various applications through trending simulation software, such as Cycle-Tempo (Toonssen et al., 2008), Simulink (Baratieri et al., 2009), and Aspen Plus (Abdelouahed et al., 2012). Gasification in DFB was detailedly explained and simulated in Aspen Plus by Abdelouahed et al. (2012), Pfeifer et al. (2007) and Proll and Hofbauer (2008a, 2008b). Most outstanding and competitive CCS technologies, incorporated with IGCC and traditional power plants, specified as oxy-fuel combustion, DEPG (Selexol) and amine scrubbing demonstrates reduction in efficiency which are in the range of 8–12% (Charitos et al., 2010); however, high temperature CCS looping cycles work as an alternative option, with ample integration capabilities, that would limit this energy deduction of about 10–12 efficiency points (Abanades et al., 2007; Lara et al., 2013). Calcium looping cycles offer a benefit of the reversible decomposition of CaCO3 along with a low sorbent cost; simultaneously, the requirement for the syngas treatment is avoided, hence CCS integration with a regular power plant significantly increases overall efficiency and reduces energy penalty 6–8%, additionally CO2 capture cost is reduced by 3–5% while the CO2 capture efficiency is increased up to 90% (Hawthorne et al., 2009; Lara et al., 2013; Romeo et al., 2008). In addition, capturing CO2 in situ in a DFB has the following advantages: (1) the calcium looping process generates additional heat, which can be effectively used in the steam cycle, thereby increasing the power of the power plant, (2) CO2 capture significantly reduce the CO2 emissions at a low cost per ton CO2 (Romeo et al., 2008). Schuster and co-workers evaluated a 10 MW gasification power plant integrated with a GT using Aspen Plus (Schuster et al., 2001). Romeo et al. (2010) used the exergy analysis, as a method for coal power plants using the calcium looping cycle and demonstrated an optimal window configuration for the integration of the calcium looping cycle into the conventional power plant for CCS.

The current study aims to develop a suitable simulation model for Calcium Looping Gasification with Combined Cycle (CLGCC) using Aspen Plus, in order to explore the influences of varying gasification temperature and pressure on the composition of H2-rich syngas and to encompass both the energy and exergy distribution considering; A detailed investigation on energy and exergy analyses have been performed to evaluate the losses during the process of CLGCC. More intensive calculation methods have been used while carefully considering every input and useful outputs of the system.

Section snippets

Process description

The CLGCC power generation schema mainly consists of eight principal blocks in sequence, i.e., gasifier, combustor, air separation unit (ASU), heat recovery steam generator (HRSG), gas cleaning (scrubber), de-sulfuring unit (Selexol), sulfur recovery unit (Claus), GT and steam turbine (ST). The block diagram of CLGCC is shown in Fig. 2.

As it can be seen, CLGCC is composed of the DFB system, a gasification unit and a combustor unit. The gasification unit is fluidized with steam and recirculated

Simulation of CLGCC system

Aspen Plus® software was used to make the simulation model for both of the comparison cases, being CLGCC and IGCC, whereby using Peng Robinson equations of state with Boston-Matias modification (PR-BM) (Proll and Hofbauer, 2008a). RGibbs reactor is used to model the DFB unit (Gasifier and Combustor), which is considered as a core unit of CLGCC. The flowsheet of the CLGCC is shown in Fig. 3. The coal quality is highly dependent on its composition and heating value, while due to its inconsistency

Exergy analysis

On the basis of 2nd law of thermodynamics, exergy is defined as maximum obtainable work that can be achieved through the equilibrium of the system with its reference surroundings (ambient conditions) (Dincer and Rosen, 2007). Exergy is destructive when the system's processes are irreversible, while for exergy conservation system's processes should be reversible. Exergy can be divided into kinetic exergyExrKn, chemical exergy ExrCh, potential exergyExrPt and physical exergy ExrPh (Li et al., 2019

Results and discussion

The simulation model results have been verified in two steps. The first part is to validate the Aspen Plus model with the experimental data, and the second part is to establish an exergy and an energy analysis of whole plant. The simulation model results are validated by calculating the root mean square error (RMSE) of H2-rich syngas compositions such as H2, CO, CO2, and CH4, using the experimental data reproduced from Wang et al. (2014). The RMSE can be calculated byMeanSquareError(MSE)=1Ni=1N

Conclusion

In this study, the thermodynamic investigation of CLGCC has been performed, representing the gasification reactor along with three reactors in sequence (RYIELD reactor and two RGIBBS reactors) by using Aspen Plus simulation software. In this work, Shenmu bituminous coal was gasified in DFB by CaO as CO2 absorber using fluidized bed technology (conventional gasification). The effects of operating parameters (gasification temperature and pressure) were validated with the experimental data. Then,

Declaration of Competing Interest

None.

Acknowledgment

We would like to thank and acknowledge the financially support provided by the National Key R&D Program of China (2019YFE100100-05).

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