Research paperThermodynamic analysis and optimization of an air Brayton cycle for recovering waste heat of blast furnace slag
Introduction
China's iron and steel industry faces huge pressure in energy-saving and environmental protection [1], [2], [3], [4], [5], [6]. A lot of waste heat is generated in steel-making process, so the waste heat recovery is one of the keys to save energy and reduce emission [7], [8]. For example, a mass of blast furnace (BF) slag is produced which belongs to high grade thermal energy and contains large amount of waste heat [9]. In 2010, China's iron and steel industry produces 200.67 million tons of BF slag which contains a lot of waste heat [10]. However, the most BF slag is treated by water granulation which recovers little waste heat as hot water. Dry slag granulation (DSG) provides a more efficient technology to recovery the waste heat of slag [11]. In DSG technology, the molten BF slag is atomized on a revolving disk, and then generated droplets are cooled and solidified by using air, and the waste heat is recovered as hot air [12], [13]. The hot air can be used in hot blast stove as combustion air or steam power plant to generate electricity.
Air Brayton cycle is firstly proposed to recovery waste heat of gas turbine in order to improve its performance [14], [15]. As gas turbine's exhaust contains a lot of waste heat, recovery of this waste heat can improve energy efficiency of gas turbine. For combined cycle power plant (CCPP), gas turbine's waste heat is recovered in steam generator, and then the steam can generate power in steam turbine. CCPP can be replaced by dual gas turbine combined cycle (DGTCC) which uses air Brayton cycle to recovery waste heat of exhaust. Compared with CCPP, DGTCC has characteristics in simple configuration, small size and absence of water [16]. Air Brayton cycle can also be used as a heat recovery equipment at high temperature furnace. Korobitsyn [17] proposed an air Brayton cycle as a heat recovery unit of a glass-melting furnace to generate power, and its efficiency is 26% while the pre-heated air can be used as combustion-supporting air in the furnace. Vittorio and Valerio [18] analyzed the performance of a solar plant operating with cylindrical parabolic collectors and an air Brayton cycle. They indicated that the solar plant has a good performance.
Finite time thermodynamics (FTT), as a branch of modern thermodynamics, has developed quickly in recent decades. FTT is a useful theory in performance analyses of various thermodynamic cycles [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. With consideration of irreversibilities in flow, mass and heat transfer, the results obtained by using FTT are closer to those of the real devices. FTT can also be well used in the performance analysis of direct and reverse Brayton cycles [24], [26], [27], [28], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. With consideration of pressure drop irreversibility, Radcenco et al. [30] established an open simple Brayton cycle's FTT model, and analyzed the effects of compressor inlet pressure drop on the cycle's efficiency and power output.
In order to improve energy efficiency of iron and steel industry, this paper firstly proposes an air Brayton cycle with dry slag granulation (DSG) equipment. In this way, the hot air recovered in DSG can be used in the air Brayton cycle to generate power. The performance of the air Brayton cycle is analyzed based on FTT. With consideration of pressure losses, the modeling of the air Brayton cycle is close to real device, and the results obtained can provide theoretical guidance for the air Brayton cycle applied in real device recovering waste heat of slag.
Section snippets
System description
The system layout is shown in Fig. 1. It includes a DSG equipment and an air Brayton cycle. In the DSG equipment, the molten BF slag is atomized on a revolving disk in the rotating cup, and then the droplets generated are quenched quickly in the rotating cup and vertical fluid bed by using blast air. The process produces solid slag granules and recovers hot air. The recovered hot air, as heat source of the air Brayton cycle, enters the heat exchanger. In the air Brayton cycle, working fluid
Performance analyses
The main purpose of this paper is to determine the optimal mass flow rate (compressor inlet relative pressure drop) and the optimal pressure ratio of the cycle which lead to the maximum power output and the maximum heat recovery efficiency. The similar studies for various open direct and reverse Brayton cycles were performed in Refs. [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44].
The compressor inlet pressure drop is:where ρ0 is density of
Numerical examples
The influences of compressor pressure ratio β1, compressor inlet relative pressure drop ψ1, and temperature of waste heat T5 on the cycle's net power output, heat recovery efficiency and thermal efficiency are examined by using numerical examples.
In the calculations, parameters are set as following: the range of the relative pressure drop of the compressor inlet is 0 < ψ1 ≤ 0.14, the range of the compressor pressure ratio is 1 < β1 < 20, the efficiency of turbine is ηt = 0.95, the contraction
Conclusion
In order to save energy in iron and steel industry, an air Brayton cycle for recovering waste heat of BF slag is proposed in this paper. A finite time thermodynamic model of the air Brayton cycle is established. The performance of the air Brayton cycle, including power, thermal efficiency and heat recovery efficiency, are analyzed by using numerical examples. The results show that the maximum power and heat recovery efficiency can be reached by adjusting compressor inlet relative pressure drop
Acknowledgements
This paper is supported by the National Key Basic Research and Development Program of China (‘973’ Program) (Grant No. 2012CB720405) and the National Natural Science Foundation of China (Grant No. 10905093). The authors wish to thank the reviewers for their careful, unbiased and constructive suggestions, which led to this revised manuscript.
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