Elsevier

Energy

Volume 174, 1 May 2019, Pages 478-487
Energy

A modified Allam cycle without compressors realizing efficient power generation with peak load shifting and CO2 capture

https://doi.org/10.1016/j.energy.2019.01.165Get rights and content

Highlights

  • A modified Allam cycle was studied with combustion product and CO2 as working medium.

  • Cycle system is simplified with higher turbine back pressure to eliminate compressor.

  • The cycle integrates power generation with CO2 capture and energy shifting.

  • Equivalent net efficiency is adopted considering 1/4 ASU consumed off-peak power.

  • Equivalent net efficiency is 48.05% with turbine inlet parameters of 30 MPa/900 °C.

Abstract

A modified Allam cycle (Allam-Z cycle) with a simpler system was proposed and investigated using NG (natural gas)/O2 combustion products mixing with the circulation CO2 as the working medium for power generation with high efficiency, zero CO2 emission and peak load shifting. The modifications are that all the working media are pumped to high pressure by pumps instead of compressors, the cold energy of both liquid oxygen and LNG is used for degrading the cooling water for CO2 liquefaction and a set of regenerative heat exchangers are arranged for turbine exhaust heat recovery. The influences of turbine parameters on the performances of the cycle were investigated. The comparison was performed under the conditions of condensation temperature of 30 °C, turbine inlet pressure of 30 MPa, inlet temperature of either 700 °C or 900 °C and the turbine outlet pressures of Allam-Z cycle and Allam cycle are 7.21 MPa and 4 MPa respectively. The results show that the output power efficiency and the equivalent net efficiency of the Allam-Z cycle with full CO2 capture are 43.64% and 40.83% respectively or 50.87% and 48.05% respectively, which are 2.15% or 2.96% higher than those of the Allam cycle under the same condition.

Introduction

The application of CCS (carbon capture and storage) technology in power generation for reduction CO2 emission into the atmosphere is considered as the main technology to solve the problem of climate change [1,2]. The biggest problem for CO2 capture in traditional coal power plants is the huge economic cost on complex technical processes with massive power consumption and it also needs a lot of chemicals [3,4]. In a traditional coal power plant with CO2 capture, the carbon dioxide composition of flue gas is decreased by about 65% while the thermal efficiency will drop around 18%. Obviously, oxy-fuel combustion has more advantages than traditional combustion since the CO2 content is concentrated with oxy-fuel combustion and can be easily separated from the mixture by conventional cooling technologies [5], therefore it provides a simple and valuable measure for CO2 capture. Many scholars and experts performed economic and technical analysis on the operation of oxy-fuel combustion power plants [[6], [7], [8], [9], [10], [11]] in recent years, and got a conclusion that oxy-fuel combustion power plants are feasible with environment-friendly features.

Beside the emissions of greenhouse gases, the limited resources and increasing global energy demand compel to reform energy structure through improving energy efficiency and developing renewable energies. Some new thermal cycles used in solar energy, biomass energy and waste heat have been proposed or studied [[12], [13], [14], [15], [16]]. The supercritical or transcritical carbon dioxide power cycles with simple loop and high thermal efficiency have captured increasing attention in recent years [17]. Carbon dioxide is a natural working fluid, nontoxic and nonflammable, and has good thermophysical properties with fairly high heat transfer coefficient. Supercritical or transcritical carbon dioxide power cycles have been applied in waste heat [18], solar energy [19] and nuclear energy [20] in different occasions. Zhang and Lior [21] proposed a CO2 and H2O mixture Rankine cycle and CO2 Brayton cycle combined power cycle using LNG and O2 as fuel and oxidant, respectively. Although the thermal efficiency of this power cycle is higher than 50%, the cycle system with both gas turbine and steam turbine loops is rather complicated. Purjam et al. [22] studied a supercritical carbon dioxide cycle and investigated the impacts of some important parameters on cycle efficiency in different temperature heat resources. Chen et al. [23] and Wu et al. [24] proposed and studied a novel LNG/O2 combustion gas and steam mixture cycle (GSMC) which is supercritical Rankine cycle using H2O/CO2 mixture working fluid. In the GSMC system, the CO2 is liquefied and captured by cryogenic liquids of both LNG and oxygen. In order to solve the problem of excessive circulation operation pressure of supercritical carbon dioxide cycle, Jeong and Yong [25] investigated a supercritical CO2–Xe and CO2–Kr mixture Brayton cycle at variable critical points. The results show that the mixture cycle can achieve higher thermal efficiency than S-CO2 cycle with lower operation pressure. Amann et al. [26] modified natural gas fueled combined cycle into O2/CO2 power cycle to realize CO2 capture. Zhang et al. [27] presented a novel combined gas and steam power cycle using cryogenic LNG and liquid oxygen to cool the inlet air and the results show a slightly higher thermal efficiency in comparison with the original cycle.

With the rapid development of China's economy, the increasing gap of peak-valley of power grids is becoming hot issue. At present, peak load regulation of coal fired power units is still the main means of peak shaving for power grids in China [28]. The peak load regulation by coal fired power units has huge influence on its service life and also reduces its thermal efficiency. Since the application of hydraulic pumped storage stations are limited by geographic conditions, while the other large-scale energy storage approaches such as air compression and air liquefaction are still under development or lack of economic justification, there are urgent demands of large-scale energy storage or shifting for the modern and future smart grid operation [29,30]. In this paper the energy storage or shifting is merged with the oxy-fuel combustion technology for power generation with CO2 capture.

Allam et al. [31,32] proposed a novel trans-critical carbon dioxide power cycle (Allam cycle) that utilizes combustion products and recirculation CO2 mixture as working fluid. Allam cycle adopts oxy-fuel combustion technology with little nitrogen in combustion to realize nearly zero NOx emission, to overcome the deficiency of high NOx emission of the conventional LNG fueled power plants. Allam cycle can dramatically achieving higher thermal efficiency compared to steam Rankine cycle and Brayton cycle.

It is apparent that the thermal efficiency of a power cycle depends mainly on the cycle operation parameters of working medium especially the turbine inlet pressure and temperature. Adopting high parameters of supercritical or ultra-supercritical is becoming the trend in the modern and future development of power plants [33]. With the rapid development of material technology and cooling methods, the biggest obstacle of supercritical and ultra-supercritical power plant is going to be eliminated [34,35]. To improve the thermal efficiency, the turbine inlet parameters in the Allam cycle [31,32] are about 30 MPa and 1150 °C respectively. The Allam cycle uses gaseous fuel with oxygen combustion products with the addition of circulation CO2 as working fluid, and the liquid or solid fuel such as coal or biomass should be converted to gaseous fuel before combustion. Liquefied natural gas (LNG), as a clean energy, with considerable cold energy and high calorific value, is widely used as fuel in peak shaving power generation plant and should be utilized first adopting CCS with the oxy-fuel combustion technology from the economic consideration.

The turbine backpressure in the Allam cycle is set considerably lower than the critical value of CO2 in consideration of the proper turbine exhaust temperature for the recuperating heat exchangers. Thus a set of compressors with intercooling are used to raise the pressure of working medium for condensing by the cooling water which makes the Allam cycle system rather complicated. In this paper, with the lower turbine inlet temperature (≤900 °C) cases without cooling approach or with only static vane cooling in turbine first stage, a simplified but still efficient power cycle based on the Allam cycle (thereafter referenced as the Allam-Z cycle) is proposed and studied for power generation with peak load shifting and CO2 capture.

Section snippets

Main circulation loop

As shown in Fig. 1, the distinguishes of the Allam-Z cycle with the Allam cycle are that the turbine backpressure is around the supercritical value rather than the lower one, thus the compressors can be eliminated and all the working media can be raised to high pressure by pumps instead of compressors; the cold energy of both LNG and liquid oxygen is used for declining the cooling water temperature in a heat exchanger (HX5) and a set of other regenerative heat exchangers are properly arranged

Parameter conditions and constraints

The thermodynamic properties of pure or mixture CO2 and H2O at vapor, liquid and two phase states data were calculated by the software of REFPROP 8 based on the NIST [39]. The inlet and outlet parameters of each component of the Allam-Z cycle are labeled subscripts consistent to the state points in the Fig. 1. Then the thermodynamic models of the cycle system and the component equipment are established with governing equations of conservation laws of mass and energy. By thermodynamic analysis

Impacts of turbine inlet parameters

Fig. 3 shows variations of the output power efficiency and the equivalent net efficiency of the Allam-Z cycle with turbine inlet temperature and pressure. Since the differential efficiency (ηel,outηeq,net) is a constant, the trend curves of the equivalent net efficiency and the output power efficiency are plotted with the same curves but different ordinate scales. Fig. 3(a) shows the variation curves of both efficiencies versus the turbine inlet temperature at constant turbine inlet pressure

Conclusions

Under the turbine inlet temperature not higher than 900 °C, the Allam-Z cycle with higher turbine back pressure to eliminate compressors was investigated using combustion product and circulation CO2 as working medium aiming at high efficiency power generation, nearly zero NOx emission, peak load shifting and CO2 capture. The conclusions are as follows:

  • 1)

    The output power efficiency increases with the increase of turbine inlet temperature at constant turbine inlet pressure. The cycle efficiency

Acknowledgement

This work is supported by the National Nature Science Foundation Program of China (No. 51776035).

References (39)

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