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Energy–environment–economy evaluations of commercial scale systems for blast furnace slag treatment: Dry slag granulation vs. water quenching
Applied Energy 171 (2016) 314–324
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Energy–environment–economy evaluations of commercial scale systems for blast furnace slag treatment: Dry slag granulation vs. water quenching Hong Wang a,b, Jun-Jun Wu b, Xun Zhu a,b,⇑, Qiang Liao a,b, Liang Zhao b a b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
h i g h l i g h t s A prototype dry slag granulation system was proposed and designed in commercial scale. The input and output flows of each slag-treatment system were identified. Life cycle assessment on the dry slag granulation method and water quenching methods was carried out. Economic costs and returns during the lifetime of slag were calculated.
a r t i c l e
i n f o
Article history: Received 8 August 2015 Received in revised form 15 March 2016 Accepted 17 March 2016
Keywords: Blast furnace slag Slag treatment Dry slag granulation Water quenching Life cycle assessment
a b s t r a c t The high-temperature blast furnace slag is conventionally treated by water quenching (WQ) method with enormous waste heat unrecovered. To address this issue, dry slag granulation (DSG) technology has been proposed to recover the waste heat from molten slag. Before commercial implementation, the sustainability and feasibility of the DSG should be well assessed. In this study, life cycle assessment (LCA) is conducted on a designed DSG prototype system in commercial scale. The environmental sustainability and economic benefit of DSG are evaluated and compared with the WQ systems. The LCA results reveal that the DSG can potentially reduce the energy and resource consumptions by 150 kg-coal-eq/t-slag and 1547 kg/t-slag, respectively. The analysis on environment impact also clarifies that the DSG is an environmentally friendly method for slag treatment. Furthermore, the DSG method represents a possibility to recover the heat from high-temperature blast furnace slag and turn it into a valuable material with a profit of 92.9$/t-slag. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The latest Adoption of Paris Agreement claimed the global response to the climate change by keeping the temperature increase below 2 °C [1]. To this end, a significant reduction in carbon dioxide emission is required, potentially resulting in more stringent emission standard on industries. As one of the most energy-and carbon-intensive industries, iron and steel manufacturing faces more grand challenge and thus has great motivation to develop and implement low carbon emission technologies [2]. One of the effective routes is to develop high-temperature waste heat recovery technologies [3]. It was estimated that about 20–50% input energy ⇑ Corresponding author at: Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China. E-mail address: [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.apenergy.2016.03.079 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
was lost as waste heat in the form of off-gas, molten slag, cooling water, etc. [4]. In the past decades, some advanced technologies, for example, top gas recovery turbine unit (TRT), coke dry quenching (CDQ) and LT-Purification and recovery (LT-PR) of converter gas, have been developed to recover heat from off-gas, which lower the energy consumption and waste emission [5,6]. However, blast furnace slag (BFS) with a large amount of heat more than 2000 TJ/ year has still been untapped [7], representing an possibility of further energy conservation in the iron and steel industries [8]. Nowadays, the BFS is usually treated by water quenching (WQ) methods including the open circuit process (OCP) [9], INBA [10], TYNA [11], RASA [12], etc. During water quenching process, highpressure water is employed to shatter the molten slag. However, the water quenching methods not only fail to recover the hightemperature waste heat but also consume a huge amount of fresh water by the evaporation of 1000–1500 l of water per ton of slag [13].
H. Wang et al. / Applied Energy 171 (2016) 314–324
As an alternative sustainable technology, dry slag granulation (DSG) with air as the cooling medium was proposed to recover the sensible heat and to reduce freshwater consumption by Berger in 1930 [14]. Since it was coined, various DSG techniques such as rotary drum [15], air blast [16] and centrifugal granulation [17,18] have been developed for slag treatment. In terms of the desirable simplicity, reliability and efficiency of equipment, the centrifugal granulation has been regarded as the most promising DSG technique to deal with the molten slag. The original centrifugal granulation system consisted of two successive fluidized beds, which was firstly designed by Pickering [19] in 1985. Conceptually, this DSG technology mainly involved two strategies: (1) slag granulation and (2) heat exchange. Good granulation producing small particles with high surface-tovolume ratio is crucially important for successful heat recovery. Mizuochi and his co-workers [17] investigated the influence of operating conditions on particle size. Another work by Mizuochi et al. [20] found that the vaned-atomizer inhibited the generation of large particles. Yu et al. [21–24] experimentally studied the centrifugal granulation mechanism of molten slag and the results showed that ligament formation was preferable for slag particle production. More recently, Zhu et al. [25,26] reported the mechanism of fibers formation during centrifugal granulation. With respect to the heat exchange of molten slag, the key challenge is to develop heat exchanger with high heat recovery efficiency and fast cooling rate of slag particles for high glassy production. Yu et al. [27,28] reported a tubular heat exchanger to produce hot water, whose the heat recovery rate ranged from 40% to 90%. Later, their works involved the glassy content inside slag particles [29]. Up to date, Commonwealth Scientific and Industrial Research Organization (CSIRO, Australia) has achieved outstanding progress in heat recovery, covering the design, development and scale-up of DSG in 2002 [30]. Based on their two-step conception, the slag can be cooled down to about 50 °C with hot air up to 600 °C. So far, despite the significant progress in DSG has been made, most of the researches are focused on the innovation of apparatus design and technical process as well as the optimization of operational conditions at laboratory scale, and the scaled-up commercial is still hardly to see. No works has been reported on the systematical estimation of the DSG technology and its advantages over WQ technique. Doubts still remain about the feasibility of DSG in commercial application for slag treatment. Therefore, a systematically and comprehensive evaluation is an indispensable and prerequisite mission for promoting the development and application of DSG technology and reducing the risk of scale-up failures. Harry E. Teasley firstly conceived a resource and environment profile analysis to manage package functions for The Coca Cola Company in 1969 [31]. The promoted analytical scheme was then developed into the world-famous life cycle assessment with the consistent efforts by Society of Environmental Toxicology and Chemistry (SETAC) [32]. It served as a tool to assess resource consumption and environmental impacts throughout the product’s lifetime [33]. This approach has been well applied to evaluate the environmental sustainability and economic affordability of various scenarios such as passenger vehicles [34], buildings [35], hydrogen production [36], and biofuels [37]. Equally important, of great interests is the discrepancy in environmental impacts and economic cost of blast furnace slag treated differently. From the life-cycle perspective of BFS, the implementation of WQ method for slag treatment involves with fresh water, electricity and coal [38]. While the DSG method only related to electricity consumption caused by the usage of granulator and fans. Therefore, a trade-off analysis between DSG and WQ methods is indispensable. The aim of present work is to implement the life cycle assessment (LCA) on the sustainability of DSG technology and thus provide a consolidated proof for the feasibility of DSG system.
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The centrifugal-granulation-based DSG system is designed in commercial scale based on existing research results and then the environment impact and economic benefits are identified. Meanwhile, a comprehensive comparison between DSG and the existed WQ methods is made to enable the selection of the optimal slagtreatment method. 2. System description In this study, the LCA is performed to analyze the sustainability of two types of slag treatment methods i.e. DSG and WQ. The sustainability in this study is related to energy consumption, resource consumption, environment emission and economy. The WQ systems (including OCP, INBA, TYNA, and RASA) are mature in industrial application. The process of slag treatment by WQ systems is depicted principally in Fig. 1a. It involves the slag granulation by high-pressure water, slag splitter from water, slag transmission and slag drying. The WQ systems are well commercialized and adopted to treat slag worldwide. On the contrary, the DSG system is still at the stage of research. To bridge the gap between WQ in commercial scale and DSG in laboratorial scale, a prototype DSG system is proposed based on three-step process (Fig. 1b). In the first stage, the slag is transmitted into the storage silo and then distributed into several granulating apparatuses that meets the slag capacity of local iron and steel company. The slag is granulated into small particles and finally cooled down below 1300 °C during flight. In the second step, the slag is then charged into a fluidized bed where the air cools the slag down to 800 °C. In the final step, the slag is continuously cooled down to 100 °C, which is used as the feedstock for cement production eventually. Especially, as the commercial scale DSG system is designed through theoretical calculation, some assumptions were made including: (1) Slag granules diameters are within 2 mm. (2) Final granules produced by DSG have the same glassy content with that produced by WQ methods. (3) Heat loss from slag granules is ignorable. Table 1 shows the detailed operational parameters of the DSG system. One can find that this system is designed with capability of treating slag at a rate up to 18 t/h (or 5 kg/s). A disc atomizer with 1.2 mm in diameter is utilized [40] and the granulating chamber is 3.2 m in diameter to assure that the slag particles surface solidify before impacting on the wall of chamber [41]. During the operation in quasi-steady state, the rotary speed maintains at 2000 rpm, and the resulted electricity is calculated as 500 W [42]. The flowrates of cool air at ambient for fluidized bed and spouted bed are 5.699 m3/s and 5.739 m3/s, respectively. The corresponding power consumptions are 49.0 kW and 41.2 kW with fan efficiency about 81.5% and 80.5% accordingly. 3. Life cycle assessment Life cycle assessment expands the key social actors’ responsibility to include environment, economy implications during the entire life cycle of product, process and system [43]. It involves the cradle-to-grave analysis, which helps identify the evident impacts on environment for different process. And it enables policy makers or social actors to figure out the optimum process, as well as possible improvement. Generally, midpoint and endpoint methodologies are available for LCA implementing. Herein, midpoint methodology is more suitable for present study because of its capability to demonstrate DSG’s sustainability in more specific environmental categories. Thus CML 2001 method is chosen to perform the assessment. Addi-
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Fig. 1. Schematic diagram of slag treatment system: (a) WQ system and (b) DSG prototype system (modified from [39]).
Table 1 Operational parameters for DSG system. Parameters
Value
Unit
Slag flow Slag flowrate Slag inlet temperature
5 1550
kg/s °C
Air flow Air flowrate Air inlet temperature
12.52 20
m3/s °C
Power flow Fan Granulator
90.2 500
kW W
tionally, EPID 2003 is also adopted to validate the robustness of the environment impact of DSG. 3.1. Scope The motivation of this LCA study is to determine the energy flow, materials flow, and capital flow of the prototype DSG system. The results intend to figure out the potential environmental loads and impact. It helps identify the relevant feasibility of present prototype DSG system by making comparison with the conventional WQ methods (OCP, INBA, TYNA and RASA). The information needed in this study comprises of slag production data, slag treatment data, and the slag drying data in slag’s lifetime. However, the slag treatment data has a wide range related to system manufacturing, operation and decommissioning. Due to lack of data, the manufacturing and decommissioning stages are
not taken into consideration for both water quenching methods and DSG method. In this regard, some upstream data used in present study comes from foreign database and extraction from published literature. The rest data used for LCA is primarily obtained by theoretical calculation according to the system design. It must be pointed out that the database or literature data are taken as the first choice and some data have to be collected from similar processes. 3.2. Inventory analysis The objective of inventory analysis is to define the system boundary, and then figure out the associated inlet and outlet flows, as shown in Fig. 2. Along the slag’s lifetime, it was firstly produced in blast furnace and then treated by DSG or WQ. For DSG method, it includes slag production process and slag treatment process (Fig. 2a). In particular, the recovered hot air suitable for secondary use is considered to verify the potential benefits. Here, the hot air is employed to preheat combustion air for blast furnace [44]. The resulted yield was then taken inside the system boundary. For WQ methods, it comprises slag production, slag treatment and slag drying processes (Fig. 2b). It should be pointed out that for slag treatment process of either DSG or WQ, the manufacturing and decommissioning stages are not accounted due to the lack of information concerning facilities material demand, electricity demand and so on. Also, the final use of slag for cement production is outside of the system boundary. The LCA is conducted in accordance with the methods presented by ISO 14041 [45]. Data related to input (raw materials,
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Fig. 2. System boundary of the slag disposal processes: (a) DSG method and (b) WQ method.
electricity, transport) and output (products, emissions to air, land and water) are collected in this study. And the collected data are standardized to define as ‘‘kg input material/t slag” or ‘‘kg output material/t slag”. As the DSG process has never been promoted for commercialization scale, the associated data is not available for current prototype system. Thus the data is primarily obtained by calculation during the system design. And all the relevant upstream data is provided by a free and open database (casestudypetvsaluempty, available in Open LCA). The data quality is assessed in the system boundary qualitatively. And the results are shown in Table 2. One can see that the data concerning the slag production and slag treatment could not meet the data quality requirements in geographic boundary due to the use of upstream data from foreign database. Nevertheless, the collected data is still of high reliability for subsequent LCA analysis.
3.3. Quantitative inventory analysis 3.3.1. Slag production According to European Slag Association [46], the blast furnace slag was eventually defined as co-product. Thus the relevant input and output flows are calculated primarily by energy allocation method. Especially, for DSG system, the inventory list is acquired based on the assumption that the heat recovery efficiency is maintained at 65%. The main data for preliminary slag production process is reported in Table 3, including resource consumption and waste emissions. Here, one can see that WQ system requires more fossil fuels than DSG system. This is because the DSG method employs recovered hot air to preheat the combustion air in blast furnace. The required fuel consumption for heating the furnace can be
Table 2 Data requirements verification. Process
Unit process
Collected data
Database use
Temporal boundary
Geographic boundary
Technical boundary
DSG
Slag production Slag treatment
Extracted & calculated Calculated
O O
O O
X X
O O
WQ
Slag production Slag treatment Slag drying
Extracted Extracted Extracted & calculated
O O –
O O O
X X O
O O O
Note: O, satisfactory; X, dissatisfactory; –, not relevant.
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Table 3 LCA input and output data for slag production stage. Indicators
Slag treatment systems WQ
a b
a
DSG (g = 65%)b
Energy consumption (kg/t)
Standard coal
222.26
102.07
Resource consumption (kg/t)
Iron ore Coal Crude oil Natural gas Dolomite Limestone
804.0 123.21 75.84 4.47 1.79 127
804.0 60.01 31.4 2.56 1.79 127
Emissions (kg/t)
CO2 SOx NOx CH4 COD NMVOC SS
528.501 0.627 1.159 1.909 0.119 0.452 0.878
373.779 0.241 0.987 1.518 0.061 0.226 0.432
DSG system. In contrast, the slag drying process is indispensable for WQ methods. Since the slag cannot be directly utilized for grinding due to its high water content, it’s arranged to be dried to decrease the water content to less than 2% for cement manufacturing. The energy consumption increases proportionally as the water content increases in slag [49]. Here, the lignite coal is taken to be the drying energy source by combustion and the emission is calculated according to the elemental analysis [50]. From Table 5, it can be found that the TYNA method required about 15.191 kg coaleq/t slag. While the OCP method consuming about 29.285 kg coaleq/t slag seems to be the most energy-intensive process among the studied WQ methods. Accordingly, the emissions aroused by OCP method are much more than the other WQ methods. 4. Results and discussion
Data from [47,48]. Data from [44,47,48].
effectively decreased. For each system, the input and output flows [47,48] are assumed to be consistent with each other except for the fossil fuel consumption and the related CO2 emission. 3.3.2. Slag treatment One of the major difference between DSG and WQ lies in the slag treatment stage. In Table 4, the main data for operation in WQ [38] and DSG systems were reported. It should be noted that the manufacturing and decommissioning of the slag treatment equipments are not involved in this study for the lack of data. Thus only the operational process is taken into consideration. For WQ systems, the input is related to electricity and freshwater while the DSG only related to electricity. In terms of energy consumption, the TYNA method consumes the least electricity of 2.5 kW h/t slag among all the methods. It can be seen that the involved electricity consumption of DSG is remarkably equal with that of INBA method, indicating the DSG is competitive with conventional WQ methods. It should be pointed out that the RASA method has been gradually abandoned for the newly-built blast furnace, pragmatically due to its significantly high energy consumption and system complexity. The potential in water saving is one of the prominent features of DSG. The DSG is totally free of direct water usage in this stage for its centrifuge- and air-based strategies. 3.3.3. Slag drying As stated earlier, the DSG is water-free when treating the molten slag particles and thus slag drying process is unnecessary for
Based on the LCA results of slag treated by different methods, the environmental loads as well as the environmental impact is analyzed during their own lifetime. For environmental loads, it mainly consists of electricity consumption and resource consumption. The environmental impacts of each systems are quantitatively measured in the scheme of CML 2001. The evaluation indicators are selected from six major environmental categories i.e. acidification potential (AP), globe warming potential for 100 annuals (GWP100a), eutrophication potential (EP), human toxicity potential (HTP) and ozone depletion potential (ODP). On the previous analysis, the life cycle of slag treated differently is assessed and an optimal method was selected among the slag treatment methods in the perspective of environmental sustainability and economic benefits. 4.1. Energy & resource consumption Fig. 3 demonstrates the energy input variations for slag treated by different methods. It is clear that the DSG method consumes the least energy of 103 kg coal-eq/t slag among all the methods. While, the WQ methods require energy input ranged from 238 kg coal-eq/t for TYNA to 253 kg coal-eq/t for OCP. The maximum gap in energy consumption between DSG and WQ is about 150 kg coal-eq/t slag, indicating the potential of DSG in energy saving. Considering the different life stages of slag, it can be found that slag production accounts for the largest share in the lifecycle energy consumption for each method. Also, the stage of slag drying holds the second largest share during the lifetime of slag for WQ systems. Surprisingly, the slag treatment stage involves ignorable energy input. The DSG system can operate at the energy cost of 0.6 kg coal-eq/t slag, while it provides order-of-magnitude greater
Table 4 LCA data for slag treatment systems [38]. Indicators
Slag treatment system OCP
INBA
TYNA
RASA
DSG
Energy consumption (kW h/t)
Electricity
8.0
5.0
2.5
15.5
5.0
Resource consumption (kg/t)
Coal Crude oil Natural gas Water
3.77 20.66 1.14 1200
2.36 12.91 0.71 900
1.18 6.46 0.36 800
7.31 40.02 2.21 1000
2.38 13.02 0.72 0
Emissions (kg/t)
CO2 SOx NOx CH4 COD NMVOC SS
13.60 0.074 0.075 0.102 0.027 0.041 0.210
8.50 0.081 0.047 0.064 0.017 0.026 0.130
4.25 0.040 0.0234 0.032 0.008 0.013 0.064
26.34 0.250 0.146 0.198 0.053 0.079 0.399
8.57 0.081 0.047 0.065 0.017 0.026 0.130
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H. Wang et al. / Applied Energy 171 (2016) 314–324 Table 5 LCA data for slag drying process [38,49,50]. Indicators
Slag treatment method OCP
INBA
TYNA
Energy consumption (kg/t)
Standard coal
29.285
21.686
15.191
24.406
Resource consumption (kg/t)
Coal Crude oil Natural gas
98.04 124.28 5.11
72.60 92.03 3.79
50.86 64.47 2.65
81.71 103.58 4.26
Emissions (kg/t)
CO2 SOx NOx CH4 COD NMVOC SS
416.600 2.784 3.880 0.650 0.161 0.245 1.232
308.499 2.061 2.873 0.481 0.119 0.181 0.913
216.10 1.444 2.013 0.337 0.084 0.127 0.639
347.198 2.320 3.234 0.542 0.134 0.204 1.027
benefit i.e. energy reduction up to 150 kg coal-eq/t slag. Technically, the priority of DSG to WQ in energy saving comes from two aspects. On one hand, recycling the recovered hot air gives rise to an energy input reduction dramatically. On the other hand, with air as the coolant, slag granules containing infinitesimal water content are suitable for direct use in grinding without drying. The dramatic energy saving of DSG implies a cost-effective scenario for future slag treatment. Thus it potentially represents an opportunity to further decrease the energy intensity in iron-making process compared with WQ methods. Another concern on the slag lifetime is the resource consumption. Fig. 4 depicts resource consumption difference existing in
RASA
each slag treatment system. The fundamental ingredients include iron ore, limestone, coal, crude oil, water, etc. One can find that WQ systems require tremendous water consumption within a range of 31–46% of the entire resource demand. On the contrary, the DSG system consumes no water and only air is used to cool down the slag. The OCP is the most resource-intensive method with a resource consumption of 2560 per ton slag. Whereas the corresponding resource consumption of DSG just amounts to 1013 kg per ton slag. A significant gap of 1547 kg/t slag between DSG and OCP, is largely dependent on the fresh water reduction of about average 1000 l/t slag of DSG. In addition to water, the share of each fossil fuel resource for OCP is as high as 16.7% and sharply decreased to 3.2% for DSG due to the heat recovery. The resource consumption gap between DSG and WQ methods reveals that DSG is more practical to economize the resources. 4.2. Environment impact
Fig. 3. Energy consumption of slag treatment methods in lifetime.
The environment impact from blast furnace slag treated by different methods is evaluated based on a set of six indicators. The life cycle impact (Fig. 5) shows that the OCP method is the most environmentally harmful with the highest score in every environmental category. The DSG system however enables a clean method to treat slag. Considering the contributions of different stages to each indicator, it can be asserted that the slag treatment stage still plays a minor role in the environmental indicators and the slag production stage dominates the GWP 100a and POCP while the slag drying process contributes to the largest part of the rest indicators (AP, EP, HTP and ODP). As far as the environment load variation of each system is concerned, a brief comparison is made according to the following equation:
gj ¼
Fig. 4. Resource consumption for DSG and WQ methods.
Ii;j Ii;OPC
ð1Þ
Here, g is the proportion of each system, I is the value of the impact indicators, i is the specific impact indicators, j is the slag treatment method. The results are shown in Fig. 6. The results based on nine dedicatedly sustainable indicators can be used to identify the respective status of WQ methods (OCP, TYNA, INBA, RASA) from a life cycle perspective. Among the WQ methods, The OCP method is also proved to be the most environmentally harmful. Besides, the TYNA method, however, is the most environmentally friendly. It is clear that the advantage of DSG is remarkable in every category compared with WQ methods. The indicators show a major gap for all the studied environment issues, announcing remarkable potential advantages. Herein, a detailed comparison is made between the TYNA method and DSG method, a gap is found to range from
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Fig. 5. Environment impact of slag from cradle to gate.
10.9% for POCP to 51.8% for ODP. It can be concluded that the DSG is an environment-friendly method for slag treatment. Concurrently, another LCA methodology i.e. ETIP 2003 is employed to validate the robustness of environmental impact calculated by CML 2001. Fig. 7 reports the corresponding characterization factors in the selected environmental categories. A similar profile can be seen for the comparison of each slag-treatment system except reduction in POCP. Even though, one can still conclude that the CML 2001 is successful to implement the LCA because no
sensitive variation in each sustainable indicator is presented in comparison with the results from EPID 2003. Finally, all the environmental indicators are homogenized into one-score indicator according to the following rule:
SEI ¼
X
EIIi f i
ð2Þ
where SEI is the environment impact score, EIIi is the environment impact indicator (see Appendix A Table 8), and fi is the weighting factor of the corresponding EIIi (see Appendix A Table 9 obtained
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H. Wang et al. / Applied Energy 171 (2016) 314–324 Table 6 Internal cost for OCP and DSG in life cycle (referred to per ton slag).
a b
Fig. 6. Environment impact of each system by CML 2001.
Unit
OCP
INBA
TYNA
RASA
DSG
Production cost Treatment cost Drying cost Internal costa External costb
$/t $/t $/t $/t $/t
102.5 11.2 65 178.7 5.0
102.5 7.2 48.1 157.8 4.2
102.5 4.0 33.7 140.2 3.4
102.5 20.4 54.2 177.1 4.6
77.5 6.3 – 83.8 1.3
Total cost
$/t
183.7
162.0
143.6
181.7
85.1
Calculated according to IMF Commodity Price [52]. Calculated according [53].
the DSG method is capable of reducing environmental impact in the scale of 60.09 compared with TYNA method. The impact reduction is even more significant (i.e. 121.39) compared with OCP method. Hence the sustainability of DSG is confirmed again with the straightforward one-score indicator.
4.3. Economy analysis An analysis on capital costs and returns is performed to expand the sustainability evaluations. In terms of the BF slag lifecycle economics, which involves capital flows in each stage of slag lifetime such as slag production, slag treatment and slag drying, the costs and returns are the major concerns for the enterprises planners. Therefore, it is essential to investigate the economy of each system. However, for the DSG system that is far behind the commercial run, its related human and maintenance costs are currently unavailable. Thus only the investment involved in machinery and equipment is taken into account for WQ and DSG methods. Here, the basic cost in resource consumption for machinery and equipment is defined as internal cost, given by the following equation:
Pinternal ¼
j i X X
Q ðjÞi CPðjÞi
ð3Þ
i¼1 j¼1
Fig. 7. Environment impact of each system by EPID 2003.
Fig. 8. Environment scores for different slag-treatment method.
from [51]). Fig. 8 reports the total environment impact score during the slag’s lifetime. One can find that the score of TYNA is 148.57, ranking last among the WQ methods, while the score of OCP is 209.87, ranking first among the WQ methods. So OCP and TYNA were taken as the representatives of WQ methods. It is revealed that
where Pinternal is the defined internal cost, i is the different life stage of slag, j is the types of consumed resource, Q is the quantity of each type of resource, CP is the corresponding commodity price. The requisite investment for each system is shown in Table 6. Regardless of slag disposal methods, the cost in slag production phase occupies more than 55% of its lifecycle cost. This is because of the definition of slag as co-product, which shares the energy or material input in iron-making process. Slag drying stage and slag treatment stage rank the second and third respectively. The cost for the conventional OCP method is $178.7/t slag among the WQ methods, while it is as least as $140.2/t slag for the TYNA method. The gap in slag lifetime cost can be well explained by two reason. First, the slag drying cost is strongly dependent on the water content in slag granules. That is, slag treated by OCP contains a high water content, and contrastingly the TYNA method enables the slag granules with much less water content. Second, the cost in slag treatment process varies for different systems. In fact, the slag treatment process is just concerned about the system operation stage. Therefore, the calculated cost in slag treatment slag is not sufficiently representative for actual expense in this phase. For DSG method, one can find that the final cost dramatically decreases down to $83.8/t slag, showing noticeable superiority in the investment for slag disposal. The decrease in cost is attributed to the employment of air as the coolant, producing dry granules without water content. Consequently, the slag drying process is not necessary. One the other hand, the recovered hot air is reused in blast furnace, which effectively lowers the fossil fuel consumption.
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Table 7 The costs and returns for slag disposal systems.
Unit Compulsory cost Optional cost Slag cement benefit Explicit profit Implicit profit a
OCP
INBA
TYNA
RASA
DSG (g = 65%)
$/t 104.3 79.4 73.7a 110 5.7
$/t 104.3 57.7 73.7a 88.3 +16
$/t 104.3 39.3 73.7a 69.9 +34.4
$/t 104.3 77.4 73.7a 108 -3.7
$/t 78.7 6.4 73.7a 11.4 +92.9
Obtained from [54].
Furthermore, the basic cost caused by waste emissions is defined as external cost according to the following rule:
Pexternal ¼
j i X X
0
0
Q ðjÞi CPðjÞi
ð4Þ
i¼1 j¼1
where Pexternal is the defined external cost, i is the different life stage of slag, j is the types of consumed resource, Q0 is the quantity of each type of waste emission, CP0 is the corresponding prices for emissions. According to the recommended shadow price of pollutions [53], the external cost is attained for all the systems. As shown in Table 6, the final external cost of DSG system is as lowest as $1.3/ t slag with respect to WQ methods, of which the cost ranges from $3.4/t to $5.0/t. With full consideration of the costs and returns for each system, the return is assumed to be in the form of glassy slag sold to cement plant. In order to better illustrate the economy during the slag’s lifetime, the cost is classified into two aspects, namely, compulsory cost and optional cost, which are defined as:
Pexplicit ¼ Ereturn ðEcompulsory þ Eoptional Þ
ð5Þ
Pimplicit ¼ ðEbasic þ Ereturn Þ ðEcompulsory þ Eoptional Þ
ð6Þ
The compulsory cost represents the slag production cost and the related cost arises from emissions, which is inevitable technically. While the optional cost represents the slag treatment cost and the following slag drying cost. And this kind of cost takes place once the planners choose to dispose the slag. From Table 7, one can find that the explicit profit values are always negative, giving an illusion that the slag disposal always causes monetary loss. However, in fact, most of the cost i.e. slag production cost is predetermined along with the iron-making, which is defined as basic cost here. And the basic cost in this process is equal to compulsory cost of WQ methods. According to formula (3), the implicit profit is
Fig. 9. Environment impact sensitivity of DSG.
Fig. 10. Net profit sensitivity of DSG for different heat recovery efficiency.
calculated and the results indicate that the DSG methods is the most economical routine to dispose the slag, which can implicitly bring a profit of about $92.9/t slag from the view of lifecycle.
4.4. Sensitivity analysis The previous analysis indicates that the benefits of DSG mainly originates from resource reduction in (i) slag production stage and (ii) slag drying stage. The former is largely related to the waste heat recovery; and the latter is inherently determined by the exempting of slag drying due to the water-free nature of DSG. As the environmental impact of DSG is closely relevant to heat recovery efficiency. A sensitivity analysis was performed on the environmental impact by changing the heat recovery efficiency g. The typical DSG scenario with heat recovery efficiency of 65% was taken as the base case (detailed values for each indicator are referred to Appendix A Table 8-DSG). Fig. 9 reports the environmental impact sensitivity of DSG. It can be seen that the ODP varying from +122% to 66%, is the most sensitive to heat recovery efficiency. While the least sensitive indicators include POCP and GWP 100a with a relatively small variation of +6% to 3%. Additionally, the net profit of DSG is also strongly dependent on the heat recovery efficiency. Fig. 10 demonstrates the net profit (explicit profit) variation with heat recovery for DSG system. As the heat recovery efficiency increases, the profit of DSG is increased almost linearly from 32.5$/t at heat efficiency of 10% to 2.2$/t at heat efficiency of 100%. In other words, as the heat recovery efficiency is increased by 1%, the profit would increase by about 0.4 $/t. Moreover, once the heat recovery efficiency is increased up to 95%, the explicit profit almost becomes 0$/t slag. In this case, the DSG method perfectly turns the slag into a valuable material both environmentally and economically. This result represents an opportunity to further make use of the BF slag. The present work provides a consolidated proof about the sustainability and feasibility of DSG. However, some major gaps between the theoretical prediction and the practical application are remained: (i) a slag feeding system has not been developed yet; (ii) the capacity of slag production is currently far behind the slag output in iron and steel sectors and (iii) a reliable heat exchanger is still unavailable to accomplish high recovery rate and high glassy content. These bottle-neck problems need to be further addressed to fill the gaps. Also, long-term works are needed to testify the safety, stability and reliability of DSG before commercializing this method.
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H. Wang et al. / Applied Energy 171 (2016) 314–324 Table 8 EII value of WQ and DSG.
AP/kgSO2-eq GWP 100a/kg CO2-eq EP/kg PO4-eq HTP/kg1,4DCB-eq POCP/kg formed ozone ODP/kg CFC-11-eq
OCP
INBA
TYNA
RASA
DSG
6.41 1019.18 0.67 63.28 0.68 3.33E06
2.67 788.2 0.25 47.81 0.64 2.47E06
2.17 725.02 0.22 37.74 0.62 1.73E06
3.03 831.2 0.28 55.2 0.68 2.78E06
0.9 436.19 0.14 12.04 0.54 5.61E09
Table 9 Weighing factor (fi) value of each environment indicator.
fi
AP
GWP 100
EP
HTP
POCP
ODP
0.085
0.098
0.076
0.149
0.083
0.15
5. Conclusion A LCA is carried out on the basis of a DSG prototype system designed at the yield of 18 t/h. Comprehensive comparisons between DSG system and WQ systems are conducted in different environmental categories. The potential in energy conservation and emission mitigation as well as considerable economic benefit are well confirmed, demonstrating the good sustainability and feasibility of DSG method. The major outcomes are: (1) The DSG method represents a potentially maximum reduction of 150 kg-coal-eq/t-slag and 1547 kg/t-slag in energy and resource consumption, respectively. (2) Wide gaps ranging from 10.9% to 51.8% are identified between DSG system and WQ systems in environmental impacts, showing that the DSG is a clean method for slag treatment. (3) A profit of $92.9/t-slag is confirmed for DSG method, which is remarkably higher than the conventional WQ methods with a profit of $34.4/t-slag to $-5.7/t-slag. The profit of DSG can be enhanced further by improving the heat recovery efficiency. Acknowledgements The author would like to thank National Basic Research Program of China (973 program) (No. 2012CB720403) and Chongqing Graduate Student Research Innovation Project (No. CYS14016). Appendix A See Tables 8 and 9. References [1] Adoption of the Paris Agreement. Proposal by the President. United Nations Office at Geneva (Switzerland); 12/12/2015. [2] Porzio GF, Fornai B, Amato A, Matarese N, Vannucci M, Chiappelli L, et al. Reducing the energy consumption and CO2 emissions of energy intensive industries through decision support systems – an example of application to the steel industry. Appl Energy 2013;112:818–33. [3] Quader MA, Ahmed S, Ghazilla RAR, Ahmed S, Dahari M. A comprehensive review on energy efficient CO2 breakthrough technologies for sustainable green iron and steel manufacturing. Renew Sustain Energy Rev 2015;50:594–614. [4] Villar A, Arribas JJ, Parrondo J. Waste-to-energy technologies in continuous process industries. Clean Technol Environ Policy 2012;14:29–39. [5] Li Y, Zhu L. Cost of energy saving and CO2 emissions reduction in China’s iron and steel sector. Appl Energy 2014;130:603–16. [6] Suopajärvi H, Pongrácz E, Fabritius T. Bioreducer use in Finnish blast furnace ironmaking – analysis of CO2 emission reduction potential and mitigation cost. Appl Energy 2014;124:82–93.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Energy–environment–economy evaluations of commercial scale systems for blast furnace slag treatment: Dry slag granulation vs. water quenching Hong Wang a,b, Jun-Jun Wu b, Xun Zhu a,b,⇑, Qiang Liao a,b, Liang Zhao b a b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
h i g h l i g h t s A prototype dry slag granulation system was proposed and designed in commercial scale. The input and output flows of each slag-treatment system were identified. Life cycle assessment on the dry slag granulation method and water quenching methods was carried out. Economic costs and returns during the lifetime of slag were calculated.
a r t i c l e
i n f o
Article history: Received 8 August 2015 Received in revised form 15 March 2016 Accepted 17 March 2016
Keywords: Blast furnace slag Slag treatment Dry slag granulation Water quenching Life cycle assessment
a b s t r a c t The high-temperature blast furnace slag is conventionally treated by water quenching (WQ) method with enormous waste heat unrecovered. To address this issue, dry slag granulation (DSG) technology has been proposed to recover the waste heat from molten slag. Before commercial implementation, the sustainability and feasibility of the DSG should be well assessed. In this study, life cycle assessment (LCA) is conducted on a designed DSG prototype system in commercial scale. The environmental sustainability and economic benefit of DSG are evaluated and compared with the WQ systems. The LCA results reveal that the DSG can potentially reduce the energy and resource consumptions by 150 kg-coal-eq/t-slag and 1547 kg/t-slag, respectively. The analysis on environment impact also clarifies that the DSG is an environmentally friendly method for slag treatment. Furthermore, the DSG method represents a possibility to recover the heat from high-temperature blast furnace slag and turn it into a valuable material with a profit of 92.9$/t-slag. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The latest Adoption of Paris Agreement claimed the global response to the climate change by keeping the temperature increase below 2 °C [1]. To this end, a significant reduction in carbon dioxide emission is required, potentially resulting in more stringent emission standard on industries. As one of the most energy-and carbon-intensive industries, iron and steel manufacturing faces more grand challenge and thus has great motivation to develop and implement low carbon emission technologies [2]. One of the effective routes is to develop high-temperature waste heat recovery technologies [3]. It was estimated that about 20–50% input energy ⇑ Corresponding author at: Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China. E-mail address: [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.apenergy.2016.03.079 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
was lost as waste heat in the form of off-gas, molten slag, cooling water, etc. [4]. In the past decades, some advanced technologies, for example, top gas recovery turbine unit (TRT), coke dry quenching (CDQ) and LT-Purification and recovery (LT-PR) of converter gas, have been developed to recover heat from off-gas, which lower the energy consumption and waste emission [5,6]. However, blast furnace slag (BFS) with a large amount of heat more than 2000 TJ/ year has still been untapped [7], representing an possibility of further energy conservation in the iron and steel industries [8]. Nowadays, the BFS is usually treated by water quenching (WQ) methods including the open circuit process (OCP) [9], INBA [10], TYNA [11], RASA [12], etc. During water quenching process, highpressure water is employed to shatter the molten slag. However, the water quenching methods not only fail to recover the hightemperature waste heat but also consume a huge amount of fresh water by the evaporation of 1000–1500 l of water per ton of slag [13].
H. Wang et al. / Applied Energy 171 (2016) 314–324
As an alternative sustainable technology, dry slag granulation (DSG) with air as the cooling medium was proposed to recover the sensible heat and to reduce freshwater consumption by Berger in 1930 [14]. Since it was coined, various DSG techniques such as rotary drum [15], air blast [16] and centrifugal granulation [17,18] have been developed for slag treatment. In terms of the desirable simplicity, reliability and efficiency of equipment, the centrifugal granulation has been regarded as the most promising DSG technique to deal with the molten slag. The original centrifugal granulation system consisted of two successive fluidized beds, which was firstly designed by Pickering [19] in 1985. Conceptually, this DSG technology mainly involved two strategies: (1) slag granulation and (2) heat exchange. Good granulation producing small particles with high surface-tovolume ratio is crucially important for successful heat recovery. Mizuochi and his co-workers [17] investigated the influence of operating conditions on particle size. Another work by Mizuochi et al. [20] found that the vaned-atomizer inhibited the generation of large particles. Yu et al. [21–24] experimentally studied the centrifugal granulation mechanism of molten slag and the results showed that ligament formation was preferable for slag particle production. More recently, Zhu et al. [25,26] reported the mechanism of fibers formation during centrifugal granulation. With respect to the heat exchange of molten slag, the key challenge is to develop heat exchanger with high heat recovery efficiency and fast cooling rate of slag particles for high glassy production. Yu et al. [27,28] reported a tubular heat exchanger to produce hot water, whose the heat recovery rate ranged from 40% to 90%. Later, their works involved the glassy content inside slag particles [29]. Up to date, Commonwealth Scientific and Industrial Research Organization (CSIRO, Australia) has achieved outstanding progress in heat recovery, covering the design, development and scale-up of DSG in 2002 [30]. Based on their two-step conception, the slag can be cooled down to about 50 °C with hot air up to 600 °C. So far, despite the significant progress in DSG has been made, most of the researches are focused on the innovation of apparatus design and technical process as well as the optimization of operational conditions at laboratory scale, and the scaled-up commercial is still hardly to see. No works has been reported on the systematical estimation of the DSG technology and its advantages over WQ technique. Doubts still remain about the feasibility of DSG in commercial application for slag treatment. Therefore, a systematically and comprehensive evaluation is an indispensable and prerequisite mission for promoting the development and application of DSG technology and reducing the risk of scale-up failures. Harry E. Teasley firstly conceived a resource and environment profile analysis to manage package functions for The Coca Cola Company in 1969 [31]. The promoted analytical scheme was then developed into the world-famous life cycle assessment with the consistent efforts by Society of Environmental Toxicology and Chemistry (SETAC) [32]. It served as a tool to assess resource consumption and environmental impacts throughout the product’s lifetime [33]. This approach has been well applied to evaluate the environmental sustainability and economic affordability of various scenarios such as passenger vehicles [34], buildings [35], hydrogen production [36], and biofuels [37]. Equally important, of great interests is the discrepancy in environmental impacts and economic cost of blast furnace slag treated differently. From the life-cycle perspective of BFS, the implementation of WQ method for slag treatment involves with fresh water, electricity and coal [38]. While the DSG method only related to electricity consumption caused by the usage of granulator and fans. Therefore, a trade-off analysis between DSG and WQ methods is indispensable. The aim of present work is to implement the life cycle assessment (LCA) on the sustainability of DSG technology and thus provide a consolidated proof for the feasibility of DSG system.
315
The centrifugal-granulation-based DSG system is designed in commercial scale based on existing research results and then the environment impact and economic benefits are identified. Meanwhile, a comprehensive comparison between DSG and the existed WQ methods is made to enable the selection of the optimal slagtreatment method. 2. System description In this study, the LCA is performed to analyze the sustainability of two types of slag treatment methods i.e. DSG and WQ. The sustainability in this study is related to energy consumption, resource consumption, environment emission and economy. The WQ systems (including OCP, INBA, TYNA, and RASA) are mature in industrial application. The process of slag treatment by WQ systems is depicted principally in Fig. 1a. It involves the slag granulation by high-pressure water, slag splitter from water, slag transmission and slag drying. The WQ systems are well commercialized and adopted to treat slag worldwide. On the contrary, the DSG system is still at the stage of research. To bridge the gap between WQ in commercial scale and DSG in laboratorial scale, a prototype DSG system is proposed based on three-step process (Fig. 1b). In the first stage, the slag is transmitted into the storage silo and then distributed into several granulating apparatuses that meets the slag capacity of local iron and steel company. The slag is granulated into small particles and finally cooled down below 1300 °C during flight. In the second step, the slag is then charged into a fluidized bed where the air cools the slag down to 800 °C. In the final step, the slag is continuously cooled down to 100 °C, which is used as the feedstock for cement production eventually. Especially, as the commercial scale DSG system is designed through theoretical calculation, some assumptions were made including: (1) Slag granules diameters are within 2 mm. (2) Final granules produced by DSG have the same glassy content with that produced by WQ methods. (3) Heat loss from slag granules is ignorable. Table 1 shows the detailed operational parameters of the DSG system. One can find that this system is designed with capability of treating slag at a rate up to 18 t/h (or 5 kg/s). A disc atomizer with 1.2 mm in diameter is utilized [40] and the granulating chamber is 3.2 m in diameter to assure that the slag particles surface solidify before impacting on the wall of chamber [41]. During the operation in quasi-steady state, the rotary speed maintains at 2000 rpm, and the resulted electricity is calculated as 500 W [42]. The flowrates of cool air at ambient for fluidized bed and spouted bed are 5.699 m3/s and 5.739 m3/s, respectively. The corresponding power consumptions are 49.0 kW and 41.2 kW with fan efficiency about 81.5% and 80.5% accordingly. 3. Life cycle assessment Life cycle assessment expands the key social actors’ responsibility to include environment, economy implications during the entire life cycle of product, process and system [43]. It involves the cradle-to-grave analysis, which helps identify the evident impacts on environment for different process. And it enables policy makers or social actors to figure out the optimum process, as well as possible improvement. Generally, midpoint and endpoint methodologies are available for LCA implementing. Herein, midpoint methodology is more suitable for present study because of its capability to demonstrate DSG’s sustainability in more specific environmental categories. Thus CML 2001 method is chosen to perform the assessment. Addi-
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Fig. 1. Schematic diagram of slag treatment system: (a) WQ system and (b) DSG prototype system (modified from [39]).
Table 1 Operational parameters for DSG system. Parameters
Value
Unit
Slag flow Slag flowrate Slag inlet temperature
5 1550
kg/s °C
Air flow Air flowrate Air inlet temperature
12.52 20
m3/s °C
Power flow Fan Granulator
90.2 500
kW W
tionally, EPID 2003 is also adopted to validate the robustness of the environment impact of DSG. 3.1. Scope The motivation of this LCA study is to determine the energy flow, materials flow, and capital flow of the prototype DSG system. The results intend to figure out the potential environmental loads and impact. It helps identify the relevant feasibility of present prototype DSG system by making comparison with the conventional WQ methods (OCP, INBA, TYNA and RASA). The information needed in this study comprises of slag production data, slag treatment data, and the slag drying data in slag’s lifetime. However, the slag treatment data has a wide range related to system manufacturing, operation and decommissioning. Due to lack of data, the manufacturing and decommissioning stages are
not taken into consideration for both water quenching methods and DSG method. In this regard, some upstream data used in present study comes from foreign database and extraction from published literature. The rest data used for LCA is primarily obtained by theoretical calculation according to the system design. It must be pointed out that the database or literature data are taken as the first choice and some data have to be collected from similar processes. 3.2. Inventory analysis The objective of inventory analysis is to define the system boundary, and then figure out the associated inlet and outlet flows, as shown in Fig. 2. Along the slag’s lifetime, it was firstly produced in blast furnace and then treated by DSG or WQ. For DSG method, it includes slag production process and slag treatment process (Fig. 2a). In particular, the recovered hot air suitable for secondary use is considered to verify the potential benefits. Here, the hot air is employed to preheat combustion air for blast furnace [44]. The resulted yield was then taken inside the system boundary. For WQ methods, it comprises slag production, slag treatment and slag drying processes (Fig. 2b). It should be pointed out that for slag treatment process of either DSG or WQ, the manufacturing and decommissioning stages are not accounted due to the lack of information concerning facilities material demand, electricity demand and so on. Also, the final use of slag for cement production is outside of the system boundary. The LCA is conducted in accordance with the methods presented by ISO 14041 [45]. Data related to input (raw materials,
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Fig. 2. System boundary of the slag disposal processes: (a) DSG method and (b) WQ method.
electricity, transport) and output (products, emissions to air, land and water) are collected in this study. And the collected data are standardized to define as ‘‘kg input material/t slag” or ‘‘kg output material/t slag”. As the DSG process has never been promoted for commercialization scale, the associated data is not available for current prototype system. Thus the data is primarily obtained by calculation during the system design. And all the relevant upstream data is provided by a free and open database (casestudypetvsaluempty, available in Open LCA). The data quality is assessed in the system boundary qualitatively. And the results are shown in Table 2. One can see that the data concerning the slag production and slag treatment could not meet the data quality requirements in geographic boundary due to the use of upstream data from foreign database. Nevertheless, the collected data is still of high reliability for subsequent LCA analysis.
3.3. Quantitative inventory analysis 3.3.1. Slag production According to European Slag Association [46], the blast furnace slag was eventually defined as co-product. Thus the relevant input and output flows are calculated primarily by energy allocation method. Especially, for DSG system, the inventory list is acquired based on the assumption that the heat recovery efficiency is maintained at 65%. The main data for preliminary slag production process is reported in Table 3, including resource consumption and waste emissions. Here, one can see that WQ system requires more fossil fuels than DSG system. This is because the DSG method employs recovered hot air to preheat the combustion air in blast furnace. The required fuel consumption for heating the furnace can be
Table 2 Data requirements verification. Process
Unit process
Collected data
Database use
Temporal boundary
Geographic boundary
Technical boundary
DSG
Slag production Slag treatment
Extracted & calculated Calculated
O O
O O
X X
O O
WQ
Slag production Slag treatment Slag drying
Extracted Extracted Extracted & calculated
O O –
O O O
X X O
O O O
Note: O, satisfactory; X, dissatisfactory; –, not relevant.
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Table 3 LCA input and output data for slag production stage. Indicators
Slag treatment systems WQ
a b
a
DSG (g = 65%)b
Energy consumption (kg/t)
Standard coal
222.26
102.07
Resource consumption (kg/t)
Iron ore Coal Crude oil Natural gas Dolomite Limestone
804.0 123.21 75.84 4.47 1.79 127
804.0 60.01 31.4 2.56 1.79 127
Emissions (kg/t)
CO2 SOx NOx CH4 COD NMVOC SS
528.501 0.627 1.159 1.909 0.119 0.452 0.878
373.779 0.241 0.987 1.518 0.061 0.226 0.432
DSG system. In contrast, the slag drying process is indispensable for WQ methods. Since the slag cannot be directly utilized for grinding due to its high water content, it’s arranged to be dried to decrease the water content to less than 2% for cement manufacturing. The energy consumption increases proportionally as the water content increases in slag [49]. Here, the lignite coal is taken to be the drying energy source by combustion and the emission is calculated according to the elemental analysis [50]. From Table 5, it can be found that the TYNA method required about 15.191 kg coaleq/t slag. While the OCP method consuming about 29.285 kg coaleq/t slag seems to be the most energy-intensive process among the studied WQ methods. Accordingly, the emissions aroused by OCP method are much more than the other WQ methods. 4. Results and discussion
Data from [47,48]. Data from [44,47,48].
effectively decreased. For each system, the input and output flows [47,48] are assumed to be consistent with each other except for the fossil fuel consumption and the related CO2 emission. 3.3.2. Slag treatment One of the major difference between DSG and WQ lies in the slag treatment stage. In Table 4, the main data for operation in WQ [38] and DSG systems were reported. It should be noted that the manufacturing and decommissioning of the slag treatment equipments are not involved in this study for the lack of data. Thus only the operational process is taken into consideration. For WQ systems, the input is related to electricity and freshwater while the DSG only related to electricity. In terms of energy consumption, the TYNA method consumes the least electricity of 2.5 kW h/t slag among all the methods. It can be seen that the involved electricity consumption of DSG is remarkably equal with that of INBA method, indicating the DSG is competitive with conventional WQ methods. It should be pointed out that the RASA method has been gradually abandoned for the newly-built blast furnace, pragmatically due to its significantly high energy consumption and system complexity. The potential in water saving is one of the prominent features of DSG. The DSG is totally free of direct water usage in this stage for its centrifuge- and air-based strategies. 3.3.3. Slag drying As stated earlier, the DSG is water-free when treating the molten slag particles and thus slag drying process is unnecessary for
Based on the LCA results of slag treated by different methods, the environmental loads as well as the environmental impact is analyzed during their own lifetime. For environmental loads, it mainly consists of electricity consumption and resource consumption. The environmental impacts of each systems are quantitatively measured in the scheme of CML 2001. The evaluation indicators are selected from six major environmental categories i.e. acidification potential (AP), globe warming potential for 100 annuals (GWP100a), eutrophication potential (EP), human toxicity potential (HTP) and ozone depletion potential (ODP). On the previous analysis, the life cycle of slag treated differently is assessed and an optimal method was selected among the slag treatment methods in the perspective of environmental sustainability and economic benefits. 4.1. Energy & resource consumption Fig. 3 demonstrates the energy input variations for slag treated by different methods. It is clear that the DSG method consumes the least energy of 103 kg coal-eq/t slag among all the methods. While, the WQ methods require energy input ranged from 238 kg coal-eq/t for TYNA to 253 kg coal-eq/t for OCP. The maximum gap in energy consumption between DSG and WQ is about 150 kg coal-eq/t slag, indicating the potential of DSG in energy saving. Considering the different life stages of slag, it can be found that slag production accounts for the largest share in the lifecycle energy consumption for each method. Also, the stage of slag drying holds the second largest share during the lifetime of slag for WQ systems. Surprisingly, the slag treatment stage involves ignorable energy input. The DSG system can operate at the energy cost of 0.6 kg coal-eq/t slag, while it provides order-of-magnitude greater
Table 4 LCA data for slag treatment systems [38]. Indicators
Slag treatment system OCP
INBA
TYNA
RASA
DSG
Energy consumption (kW h/t)
Electricity
8.0
5.0
2.5
15.5
5.0
Resource consumption (kg/t)
Coal Crude oil Natural gas Water
3.77 20.66 1.14 1200
2.36 12.91 0.71 900
1.18 6.46 0.36 800
7.31 40.02 2.21 1000
2.38 13.02 0.72 0
Emissions (kg/t)
CO2 SOx NOx CH4 COD NMVOC SS
13.60 0.074 0.075 0.102 0.027 0.041 0.210
8.50 0.081 0.047 0.064 0.017 0.026 0.130
4.25 0.040 0.0234 0.032 0.008 0.013 0.064
26.34 0.250 0.146 0.198 0.053 0.079 0.399
8.57 0.081 0.047 0.065 0.017 0.026 0.130
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H. Wang et al. / Applied Energy 171 (2016) 314–324 Table 5 LCA data for slag drying process [38,49,50]. Indicators
Slag treatment method OCP
INBA
TYNA
Energy consumption (kg/t)
Standard coal
29.285
21.686
15.191
24.406
Resource consumption (kg/t)
Coal Crude oil Natural gas
98.04 124.28 5.11
72.60 92.03 3.79
50.86 64.47 2.65
81.71 103.58 4.26
Emissions (kg/t)
CO2 SOx NOx CH4 COD NMVOC SS
416.600 2.784 3.880 0.650 0.161 0.245 1.232
308.499 2.061 2.873 0.481 0.119 0.181 0.913
216.10 1.444 2.013 0.337 0.084 0.127 0.639
347.198 2.320 3.234 0.542 0.134 0.204 1.027
benefit i.e. energy reduction up to 150 kg coal-eq/t slag. Technically, the priority of DSG to WQ in energy saving comes from two aspects. On one hand, recycling the recovered hot air gives rise to an energy input reduction dramatically. On the other hand, with air as the coolant, slag granules containing infinitesimal water content are suitable for direct use in grinding without drying. The dramatic energy saving of DSG implies a cost-effective scenario for future slag treatment. Thus it potentially represents an opportunity to further decrease the energy intensity in iron-making process compared with WQ methods. Another concern on the slag lifetime is the resource consumption. Fig. 4 depicts resource consumption difference existing in
RASA
each slag treatment system. The fundamental ingredients include iron ore, limestone, coal, crude oil, water, etc. One can find that WQ systems require tremendous water consumption within a range of 31–46% of the entire resource demand. On the contrary, the DSG system consumes no water and only air is used to cool down the slag. The OCP is the most resource-intensive method with a resource consumption of 2560 per ton slag. Whereas the corresponding resource consumption of DSG just amounts to 1013 kg per ton slag. A significant gap of 1547 kg/t slag between DSG and OCP, is largely dependent on the fresh water reduction of about average 1000 l/t slag of DSG. In addition to water, the share of each fossil fuel resource for OCP is as high as 16.7% and sharply decreased to 3.2% for DSG due to the heat recovery. The resource consumption gap between DSG and WQ methods reveals that DSG is more practical to economize the resources. 4.2. Environment impact
Fig. 3. Energy consumption of slag treatment methods in lifetime.
The environment impact from blast furnace slag treated by different methods is evaluated based on a set of six indicators. The life cycle impact (Fig. 5) shows that the OCP method is the most environmentally harmful with the highest score in every environmental category. The DSG system however enables a clean method to treat slag. Considering the contributions of different stages to each indicator, it can be asserted that the slag treatment stage still plays a minor role in the environmental indicators and the slag production stage dominates the GWP 100a and POCP while the slag drying process contributes to the largest part of the rest indicators (AP, EP, HTP and ODP). As far as the environment load variation of each system is concerned, a brief comparison is made according to the following equation:
gj ¼
Fig. 4. Resource consumption for DSG and WQ methods.
Ii;j Ii;OPC
ð1Þ
Here, g is the proportion of each system, I is the value of the impact indicators, i is the specific impact indicators, j is the slag treatment method. The results are shown in Fig. 6. The results based on nine dedicatedly sustainable indicators can be used to identify the respective status of WQ methods (OCP, TYNA, INBA, RASA) from a life cycle perspective. Among the WQ methods, The OCP method is also proved to be the most environmentally harmful. Besides, the TYNA method, however, is the most environmentally friendly. It is clear that the advantage of DSG is remarkable in every category compared with WQ methods. The indicators show a major gap for all the studied environment issues, announcing remarkable potential advantages. Herein, a detailed comparison is made between the TYNA method and DSG method, a gap is found to range from
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Fig. 5. Environment impact of slag from cradle to gate.
10.9% for POCP to 51.8% for ODP. It can be concluded that the DSG is an environment-friendly method for slag treatment. Concurrently, another LCA methodology i.e. ETIP 2003 is employed to validate the robustness of environmental impact calculated by CML 2001. Fig. 7 reports the corresponding characterization factors in the selected environmental categories. A similar profile can be seen for the comparison of each slag-treatment system except reduction in POCP. Even though, one can still conclude that the CML 2001 is successful to implement the LCA because no
sensitive variation in each sustainable indicator is presented in comparison with the results from EPID 2003. Finally, all the environmental indicators are homogenized into one-score indicator according to the following rule:
SEI ¼
X
EIIi f i
ð2Þ
where SEI is the environment impact score, EIIi is the environment impact indicator (see Appendix A Table 8), and fi is the weighting factor of the corresponding EIIi (see Appendix A Table 9 obtained
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H. Wang et al. / Applied Energy 171 (2016) 314–324 Table 6 Internal cost for OCP and DSG in life cycle (referred to per ton slag).
a b
Fig. 6. Environment impact of each system by CML 2001.
Unit
OCP
INBA
TYNA
RASA
DSG
Production cost Treatment cost Drying cost Internal costa External costb
$/t $/t $/t $/t $/t
102.5 11.2 65 178.7 5.0
102.5 7.2 48.1 157.8 4.2
102.5 4.0 33.7 140.2 3.4
102.5 20.4 54.2 177.1 4.6
77.5 6.3 – 83.8 1.3
Total cost
$/t
183.7
162.0
143.6
181.7
85.1
Calculated according to IMF Commodity Price [52]. Calculated according [53].
the DSG method is capable of reducing environmental impact in the scale of 60.09 compared with TYNA method. The impact reduction is even more significant (i.e. 121.39) compared with OCP method. Hence the sustainability of DSG is confirmed again with the straightforward one-score indicator.
4.3. Economy analysis An analysis on capital costs and returns is performed to expand the sustainability evaluations. In terms of the BF slag lifecycle economics, which involves capital flows in each stage of slag lifetime such as slag production, slag treatment and slag drying, the costs and returns are the major concerns for the enterprises planners. Therefore, it is essential to investigate the economy of each system. However, for the DSG system that is far behind the commercial run, its related human and maintenance costs are currently unavailable. Thus only the investment involved in machinery and equipment is taken into account for WQ and DSG methods. Here, the basic cost in resource consumption for machinery and equipment is defined as internal cost, given by the following equation:
Pinternal ¼
j i X X
Q ðjÞi CPðjÞi
ð3Þ
i¼1 j¼1
Fig. 7. Environment impact of each system by EPID 2003.
Fig. 8. Environment scores for different slag-treatment method.
from [51]). Fig. 8 reports the total environment impact score during the slag’s lifetime. One can find that the score of TYNA is 148.57, ranking last among the WQ methods, while the score of OCP is 209.87, ranking first among the WQ methods. So OCP and TYNA were taken as the representatives of WQ methods. It is revealed that
where Pinternal is the defined internal cost, i is the different life stage of slag, j is the types of consumed resource, Q is the quantity of each type of resource, CP is the corresponding commodity price. The requisite investment for each system is shown in Table 6. Regardless of slag disposal methods, the cost in slag production phase occupies more than 55% of its lifecycle cost. This is because of the definition of slag as co-product, which shares the energy or material input in iron-making process. Slag drying stage and slag treatment stage rank the second and third respectively. The cost for the conventional OCP method is $178.7/t slag among the WQ methods, while it is as least as $140.2/t slag for the TYNA method. The gap in slag lifetime cost can be well explained by two reason. First, the slag drying cost is strongly dependent on the water content in slag granules. That is, slag treated by OCP contains a high water content, and contrastingly the TYNA method enables the slag granules with much less water content. Second, the cost in slag treatment process varies for different systems. In fact, the slag treatment process is just concerned about the system operation stage. Therefore, the calculated cost in slag treatment slag is not sufficiently representative for actual expense in this phase. For DSG method, one can find that the final cost dramatically decreases down to $83.8/t slag, showing noticeable superiority in the investment for slag disposal. The decrease in cost is attributed to the employment of air as the coolant, producing dry granules without water content. Consequently, the slag drying process is not necessary. One the other hand, the recovered hot air is reused in blast furnace, which effectively lowers the fossil fuel consumption.
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Table 7 The costs and returns for slag disposal systems.
Unit Compulsory cost Optional cost Slag cement benefit Explicit profit Implicit profit a
OCP
INBA
TYNA
RASA
DSG (g = 65%)
$/t 104.3 79.4 73.7a 110 5.7
$/t 104.3 57.7 73.7a 88.3 +16
$/t 104.3 39.3 73.7a 69.9 +34.4
$/t 104.3 77.4 73.7a 108 -3.7
$/t 78.7 6.4 73.7a 11.4 +92.9
Obtained from [54].
Furthermore, the basic cost caused by waste emissions is defined as external cost according to the following rule:
Pexternal ¼
j i X X
0
0
Q ðjÞi CPðjÞi
ð4Þ
i¼1 j¼1
where Pexternal is the defined external cost, i is the different life stage of slag, j is the types of consumed resource, Q0 is the quantity of each type of waste emission, CP0 is the corresponding prices for emissions. According to the recommended shadow price of pollutions [53], the external cost is attained for all the systems. As shown in Table 6, the final external cost of DSG system is as lowest as $1.3/ t slag with respect to WQ methods, of which the cost ranges from $3.4/t to $5.0/t. With full consideration of the costs and returns for each system, the return is assumed to be in the form of glassy slag sold to cement plant. In order to better illustrate the economy during the slag’s lifetime, the cost is classified into two aspects, namely, compulsory cost and optional cost, which are defined as:
Pexplicit ¼ Ereturn ðEcompulsory þ Eoptional Þ
ð5Þ
Pimplicit ¼ ðEbasic þ Ereturn Þ ðEcompulsory þ Eoptional Þ
ð6Þ
The compulsory cost represents the slag production cost and the related cost arises from emissions, which is inevitable technically. While the optional cost represents the slag treatment cost and the following slag drying cost. And this kind of cost takes place once the planners choose to dispose the slag. From Table 7, one can find that the explicit profit values are always negative, giving an illusion that the slag disposal always causes monetary loss. However, in fact, most of the cost i.e. slag production cost is predetermined along with the iron-making, which is defined as basic cost here. And the basic cost in this process is equal to compulsory cost of WQ methods. According to formula (3), the implicit profit is
Fig. 9. Environment impact sensitivity of DSG.
Fig. 10. Net profit sensitivity of DSG for different heat recovery efficiency.
calculated and the results indicate that the DSG methods is the most economical routine to dispose the slag, which can implicitly bring a profit of about $92.9/t slag from the view of lifecycle.
4.4. Sensitivity analysis The previous analysis indicates that the benefits of DSG mainly originates from resource reduction in (i) slag production stage and (ii) slag drying stage. The former is largely related to the waste heat recovery; and the latter is inherently determined by the exempting of slag drying due to the water-free nature of DSG. As the environmental impact of DSG is closely relevant to heat recovery efficiency. A sensitivity analysis was performed on the environmental impact by changing the heat recovery efficiency g. The typical DSG scenario with heat recovery efficiency of 65% was taken as the base case (detailed values for each indicator are referred to Appendix A Table 8-DSG). Fig. 9 reports the environmental impact sensitivity of DSG. It can be seen that the ODP varying from +122% to 66%, is the most sensitive to heat recovery efficiency. While the least sensitive indicators include POCP and GWP 100a with a relatively small variation of +6% to 3%. Additionally, the net profit of DSG is also strongly dependent on the heat recovery efficiency. Fig. 10 demonstrates the net profit (explicit profit) variation with heat recovery for DSG system. As the heat recovery efficiency increases, the profit of DSG is increased almost linearly from 32.5$/t at heat efficiency of 10% to 2.2$/t at heat efficiency of 100%. In other words, as the heat recovery efficiency is increased by 1%, the profit would increase by about 0.4 $/t. Moreover, once the heat recovery efficiency is increased up to 95%, the explicit profit almost becomes 0$/t slag. In this case, the DSG method perfectly turns the slag into a valuable material both environmentally and economically. This result represents an opportunity to further make use of the BF slag. The present work provides a consolidated proof about the sustainability and feasibility of DSG. However, some major gaps between the theoretical prediction and the practical application are remained: (i) a slag feeding system has not been developed yet; (ii) the capacity of slag production is currently far behind the slag output in iron and steel sectors and (iii) a reliable heat exchanger is still unavailable to accomplish high recovery rate and high glassy content. These bottle-neck problems need to be further addressed to fill the gaps. Also, long-term works are needed to testify the safety, stability and reliability of DSG before commercializing this method.
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H. Wang et al. / Applied Energy 171 (2016) 314–324 Table 8 EII value of WQ and DSG.
AP/kgSO2-eq GWP 100a/kg CO2-eq EP/kg PO4-eq HTP/kg1,4DCB-eq POCP/kg formed ozone ODP/kg CFC-11-eq
OCP
INBA
TYNA
RASA
DSG
6.41 1019.18 0.67 63.28 0.68 3.33E06
2.67 788.2 0.25 47.81 0.64 2.47E06
2.17 725.02 0.22 37.74 0.62 1.73E06
3.03 831.2 0.28 55.2 0.68 2.78E06
0.9 436.19 0.14 12.04 0.54 5.61E09
Table 9 Weighing factor (fi) value of each environment indicator.
fi
AP
GWP 100
EP
HTP
POCP
ODP
0.085
0.098
0.076
0.149
0.083
0.15
5. Conclusion A LCA is carried out on the basis of a DSG prototype system designed at the yield of 18 t/h. Comprehensive comparisons between DSG system and WQ systems are conducted in different environmental categories. The potential in energy conservation and emission mitigation as well as considerable economic benefit are well confirmed, demonstrating the good sustainability and feasibility of DSG method. The major outcomes are: (1) The DSG method represents a potentially maximum reduction of 150 kg-coal-eq/t-slag and 1547 kg/t-slag in energy and resource consumption, respectively. (2) Wide gaps ranging from 10.9% to 51.8% are identified between DSG system and WQ systems in environmental impacts, showing that the DSG is a clean method for slag treatment. (3) A profit of $92.9/t-slag is confirmed for DSG method, which is remarkably higher than the conventional WQ methods with a profit of $34.4/t-slag to $-5.7/t-slag. The profit of DSG can be enhanced further by improving the heat recovery efficiency. Acknowledgements The author would like to thank National Basic Research Program of China (973 program) (No. 2012CB720403) and Chongqing Graduate Student Research Innovation Project (No. CYS14016). Appendix A See Tables 8 and 9. References [1] Adoption of the Paris Agreement. Proposal by the President. United Nations Office at Geneva (Switzerland); 12/12/2015. [2] Porzio GF, Fornai B, Amato A, Matarese N, Vannucci M, Chiappelli L, et al. Reducing the energy consumption and CO2 emissions of energy intensive industries through decision support systems – an example of application to the steel industry. Appl Energy 2013;112:818–33. [3] Quader MA, Ahmed S, Ghazilla RAR, Ahmed S, Dahari M. A comprehensive review on energy efficient CO2 breakthrough technologies for sustainable green iron and steel manufacturing. Renew Sustain Energy Rev 2015;50:594–614. [4] Villar A, Arribas JJ, Parrondo J. Waste-to-energy technologies in continuous process industries. Clean Technol Environ Policy 2012;14:29–39. [5] Li Y, Zhu L. Cost of energy saving and CO2 emissions reduction in China’s iron and steel sector. Appl Energy 2014;130:603–16. [6] Suopajärvi H, Pongrácz E, Fabritius T. Bioreducer use in Finnish blast furnace ironmaking – analysis of CO2 emission reduction potential and mitigation cost. Appl Energy 2014;124:82–93.
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