CHEMICAL ENGINEERING TRANSACTIONS VOL. 52, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-42-6; ISSN 2283-9216 A Sankey Diagram Approach to Quantifying Industrial Residual Energy in China Zheng Zhao, Ti Wang, Pei Liu*, Zheng Li State Key Lab of Power Systems, Department of Thermal Engineering, Tsinghua University, Beijing 10084, China liu_pei@tsinghua.edu.cn The industrial sectors are the largest energy consumers of China, accounting for more than 60 % of China’s overall energy consumption. However, a large part of energy in the industrial sector is not used efficiently, and there is great potential for enhancing energy utilization efficiency. This paper is the first work that explicitly addresses the conception of China’s industrial residual energy, which includes the definition, classification and quantification of twelve high-consuming industry sectors in China. Sankey Diagram approach is applied to analyse the current situation of IRE that can intuitively reveal the relations among specific forms and the reason why China has not utilized IRE efficiently, based on which we propose the policy suggestions. The energy flows are illustrated via Sankey Diagrams, which explicitly indicate stages with huge energy waste thus great potential for improving. Results show that the iron and steel sector has the largest potential for energy recovery, amounting to approximately 300 Mt coal equivalent per year. The energy recovery potential in each sector is further divided into three levels according to technical difficulty, namely full potential, technically recoverable (TR), and already utilized. Based on the results, prioritizing development of some energy recovery technologies is proposed, including coke dry quenching, combined cycle power generation from industrial residual gas, and power generation with low temperature waste heat during cement production, with an expected annual energy saving of 120 million tonnes coal equivalent. 1. Introduction China is the largest energy consumer around the world, and its energy utilization efficiency still has large potential for improvement. In the energy-extensive industries, China spends 15 % more energy than international advanced level with the same production value. In China, high-energy-consumption industries mainly include Smelting and Pressing of Ferrous Metals, Smelting and Pressing of Non-ferrous Metals, Manufacture of Non-metallic Mineral Products, Manufacture of Raw Chemical Materials and Chemical Products, Process of Petroleum, Manufacture of Textile and Manufacture of Paper, which totally consume 79 % of total industrial energy, 45 % of China’s total energy consumption as well (Zhou et al., 2010). Industrial residual energy, as a kind of secondary energy, refers to the pressure energy and thermal energy (both sensible and potential) produced by certain industrial processes. The residual energy is often carried by specific mediums, such as smoke, cooling water, hot production, hot slag and high-pressure gas, all of which make up a considerable heat source, namely industrial residual energy (Zhang et al., 2013). Different from most of the developed countries, China is a late starter in industrial process, and many industrial sectors have not utilized the energy resources in a best way, generating plenty of exploitable mediums. Some Chinese industries have been concentrating on proceeding industrial residual energy utilization, but the overall performance has been unknown. Although many researches have been focusing on specific techniques of utilizing industrial residual heat, there isn’t any quantitative analysis of the overall situation. In this work, we are going to illustrate the current residual energy utilization situation of industrial sectors in a quantitative way. DOI: 10.3303/CET1652007 Please cite this article as: Zhao Z., Wang T., Liu P., Li Z., 2016, A sankey diagram approach to quantifying industrial residual energy in china, Chemical Engineering Transactions, 52, 37-42 DOI:10.3303/CET1652007 37 2. Overview IRE concentrates in the industrial mediums, which contains much available energy. However, we can’t utilize all IRE because of the space limitation of industrial plants and high proportion of low-temperature mediums. For figuring out how much IRE we can use, we need to analyse the specific forms of IRE in detail. IRE can be classified into sensible heat, combustible heat and pressure energy, which behaves in vapour, liquid and solid forms. In this section we are going to illustrate the specific forms of every involved industrial sector. Specific IRE forms are shown from Table 1 to Table 5. Table 1: IRE forms of iron and steel industry Forms Process Category State of matter Temperature (°C) Hot Coke Coking Sensible heat solid 950 ~ 1,050 Coke Oven Gas Coking Sensible heat; combustion heat vapor 650 ~ 850 Coke Oven Smoke Coking Sensible heat vapor 180 ~ 230 Hot Sinter Sintering Sensible heat solid 650 ~ 800 Sintering Smoke Sintering Sensible heat vapor 150 ~ 200 Hot Pellet Pelletizing Sensible heat solid 350 ~ 500 Pelletizing Smoke Pelletizing Sensible heat vapor 100 ~ 200 Blast Furnace Slag Iron making Sensible heat solid 1,400 ~ 1,500 Blast Furnace Gas Iron making Sensible and combustible heat; pressure energy vapor 300 Stove-exhausting Gas Iron making Sensible heat vapor 200 ~ 300 Hot Steel Slag Steel making Sensible heat solid 1,400 Converter Gas Steel making Sensible heat; combustion heat solid > 1,000 Converter Smoke Steel making Sensible heat vapor 1,400 ~ 1,600 Steel-rolling Hot Blast Furnace Smoke Steel rolling Sensible heat vapor 350 ~ 400 Cooling Water All Sensible heat liquid < 100 Hot Coke Coking Sensible heat solid 950 ~ 1,050 Table 2: IRE forms of electrolytic aluminum industry Forms Process Category State of matter Temperature (°C) Electrolysis Gas Aluminum electrolysis Sensible heat vapor 100 ~ 200 Rotary Kiln Smoke Production of anode carbon Sensible heat vapor 800 ~ 1,100 Baking Furnace Smoke Production of anode carbon Sensible heat vapor 150 ~ 250 Electric Forge Furnace Gas Production of cathode carbon Sensible and combustible heat vapor 300 ~ 400 3. Quantification 3.1 Full potential Having already shown different forms of IRE, in this part, we are going to give a universal quantification criteria of full potential in terms of sensible heat, combustible heat and pressure energy. Based on the quantification standard, the Sankey Diagram of IRE full potential will be given for showing the general pattern of China. As we know, sensible heat can be measured only by calculating the enthalpy difference between two states of IRE mediums, namely upper limit and lower limit. The upper limit naturally refers to the state of IRE mediums produced by industrial processes, as has been shown in section 2. Considering the rapid development of low- temperature Rankine cycle and heat pump technology, we set 30 °C as the lower limit, which means the enthalpy higher than the state of 30 °C is the sensible heat quantity. Combustible heat quantity refers to the energy that IRE mediums can release when it is burned. Pressure energy quantity is the maximum work when high-pressure mediums drive the turbine. 38 Table 3: IRE forms of synthesis ammonia industry Forms Process Category State of matter Temperature (°C) Upward Gas Gas generation section Sensible heat vapor 320 or > 600 Downward Gas Gas generation section Sensible heat vapor 270 Blow-out Air Gas generation section Sensible and Combustible heat vapor 310 Gas Generation Slag and Debris Gas generation section Combustible heat solid Gasifier Jacket Gas generation section Sensible heat solid Shift Gas Conversion section Sensible heat vapor 90 Cuprammonium Refining sector Sensible heat liquid 78 Synthesis Gas Synthetic section Sensible heat vapor 350 Table 4: IRE forms of glass industry Forms Category State of matter Temperature(°C) Furnace Body Sensible heat solid > 150 Tin Bath Body Sensible heat solid 200 Melting Furnace Smoke Sensible heat vapor 450 ~ 550 Annealing Furnace Exhaust Air Sensible heat vapor 280 ~ 500 Circulating Cooling Water Sensible heat liquid 35 Table 5: IRE forms of other industrial sectors Industry sector Forms Category State of matter Temperature(°C) Oil Refining Oil products Sensible heat liquid 70 ~ 200 Oil Refining Smoke Sensible heat vapor < 200 Oil Refining Waste Water Sensible heat liquid < 150 Oil Refining Steam Sensible heat vapor Oil Refining Flare Gas Combustible heat vapor Cement Exhaust Sensible heat vapor < 400 Carbon Black Off-gas Combustible heat vapor Textile Dyeing Condensate Water Sensible heat liquid 100 Textile Dyeing Cooling Water Sensible heat liquid 60 Textile Dyeing Dye Vat Water Sensible heat liquid 60 ~ 70 Textile Dyeing Boiler Smoke Sensible heat vapor > 220 Textile Dyeing Calibrator's Waste gas Sensible heat vapor > 120 Paper Making Boiler Smoke Sensible heat vapor 110 ~ 145 Paper Making Hot Air Sensible heat vapor 65 Calcium Carbide Furnace Smoke Sensible heat vapor 350~600 Calcium Carbide Fused Products Sensible heat solid 2,000 Sodium Carbonate Calcinatory Gas Sensible heat vapor 120 Vitriol Sulfur Dioxide Gas Sensible heat vapor 950 Vitriol Sulfur Trioxide Gas Sensible heat vapor 400 Vitriol Oil Products of Vitriol Sensible heat liquid 110~120 Copper Refining Reverberatory Furnace Smoke Sensible heat vapor 1,200 Copper Refining Flash Furnace Smoke Sensible heat vapor 1,300 Copper Refining Converter Smoke Sensible heat vapor 1,100 Copper Refining Anode Furnace Smoke Sensible heat vapor 1,400 Copper Refining Electric Furnace Smoke Sensible heat vapor 270 Copper Refining Electric Furnace Slag Sensible heat solid > 1,200 39 Specific Formulas are given below: Sensible heat 𝑄𝑠 = 𝑀𝑖 ∫ 𝐶𝑖 𝑑𝑇 = 𝑀𝑖 × ∆𝐻 (1) 𝑀𝑖,𝐶𝑖, 𝑑𝑇 and ∆𝐻 are mass, specific heat capacity, temperature difference and enthalpy difference of IRE mediums respectively. Combustible heat: 𝑄𝑐 = 𝑀𝑖 × 𝑞𝑖 𝑚 = 𝑉𝑖 × 𝑞𝑖 𝑣 (2) 𝑀𝑖,𝑉𝑖 ,𝑞𝑖 𝑚 and 𝑞𝑖 𝑣 are mass, volume, combustible heat per unit volume and combustible heat per unit mass of IRE mediums respectively. After the calculation based on above criteria, this paper uses the Sankey Diagram to show the results of full potential, as shown in Figure 1. Figure 1: Sankey Diagram of China’s IRE full potential 3.2 Current status of utilization In this part, according to the investigation of China’s industry, we will show the Sankey Diagrams of IRE utilization of different industrial sectors. Due to space limitation, we choose iron and steel industry and synthesis ammonia as examples to show how Sankey Diagram approach works in this study, as shown from Figure 2 and Figure 3. In every Sankey Diagram, yellow parts represent the full potential, brown parts representing the technically recoverable, blue parts on behalf of already utilized residual heat, we can intuitively find the key problems of IRE recovery from these Sankey Diagrams. All the units are 10,000 t of coal equivalent. Figure 4 shows the overall utilization status of China’s industrial residual energy. 40 Figure 2: Sankey Diagram of iron and steel industry PE: pressure energy; BFG: blast furnace gas; SH: sensible heat; COG: coke oven gas; CG: converter gas; CDQ: coke dry quenching; BFS: blast furnace slag; COS: coke oven smoke; CMC: coal moisture control Figure 3: Sankey Diagram of synthesis ammonia industry CH: combustible heat; UDG: upward and downward gas; GGSD: gas generation slag and debris Figure 4: Overall Sankey Diagram of China’s industrial residual energy utilization status 41 3.3 Analysis From the gap between different stages among every industry sector, we can easily know the problems of IRE utilization. From Figure 2, one can know the sensible heat of hot pellet, hot steel slag and steel-rolling hot blast furnace, which amounts to 9 Mt of coal equivalent, can’t be utilized in certain ways (Wang et al., 2014). Meanwhile, some good using methods of IRE have been proved to be effective and feasible, such as coal moisture control technique and using hot waste water for heating (Wang et al., 2014), having not been popularized because of reforming difficulty and expensive cost, which can save at least 30 Mt of coal equivalent. As far as iron and steel industry is concerned, it’s needful to apply effective using methods into practice and develop the utilization potential of sensible heat at present. From Figure 3, one can know the sensible heat of shift gas, which amounts to around 6 Mt of coal equivalent, can only be used in one third. Meanwhile, the combustible heat of synthesis gas, which is around 4 million coal equivalent, has not been utilized (Xi, 2011). In conclusion, it’s necessary to develop the low-temperature residual energy of shift gas and burn synthesis gas for a proper purpose. 4. Conclusions Currently, there are 500 Mt of coal equivalent full potential of IRE in China, of which 400 Mt are technically recoverable, and 200 Mt have already been utilized. Among the 13 industrial sectors, iron and steel industry is the most potential, whose full potential is about 300 Mt of coal equivalent. The paper is the first work that introduces the specific forms of IRE throughout all the high-consuming industry sectors in China, before which all papers about IRE are focusing on one certain area. From the Sankey Diagrams and forms shown above, this paper reveals the current situation of IRE and intuitively shows the parts where China has not utilized IRE efficiently. Current problems of IRE mainly lie in the development of utilization potential and promotion of feasible using methods, the specific data of which are shown in Sankey Diagrams. 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