1. Application case carbon emission estimation
The material of a casting is HT150, which is produced in batches. As shown in Figure 3, the parts are thin-walled flat covers, which are installed in the front of the automobile engine. Except for the fixed screw holes and some parts with large thickness, they are all 4.5mm. This part supports and seals without load. The overall dimension of the casting is 324mm × 284mm × 87.5mm, the net weight of the casting is 4.88kg, the total mass is 6.50kg, and the rejection rate of the enterprise is 73%.
Process design: this casting belongs to the thin-walled part of ordinary grey cast iron. It is decided to adopt sand casting. The molding adopts mechanical molding. The equipment adopts multi contact high-pressure molding line. The maximum pressure strength of the upper and lower main engines is 785kn. The size of the sand box is 1000 × 1200 × 450 / 450mm3. The maximum specific pressure of the molding machine can reach 1.3mpa. Four castings are placed in each box. Without sand core, the main cavity of the casting is formed by the sand mold on the lower box, and the part of the inner hole with diameter of 72mm is formed by the hanging sand on the upper box.
Energy consumption of process system is the sum of power consumption of equipment used in each process.
The core of the casting is not used, so the main auxiliary material is molding sand. The energy consumption includes the power consumption of the equipment required in the processing of molding sand, as calculated in formula (2); the energy consumption of various raw materials in molding sand, such as raw sand, clay and coal powder, in the mining, transportation and processing stages is calculated in formula (3), and the used sand is used as the circulating material in the system. According to the literature, the energy consumption is not calculated. With reference to simapro 7.1 database, literature  and actual investigation data, energy consumption of raw sand and bentonite in each stage of mining, transportation and processing of new sand is discussed.
The energy consumption of raw material system is the largest part of the energy consumption of casting system, including the energy consumption of pig iron smelting process and cupola charge preparation. The material flow and energy flow are shown in the figure.
According to the actual cupola burden ratio of the enterprise, the bill of materials per kilogram of cast iron is shown in Table 3; according to the productivity of each process in the pig iron production process, the quality of raw materials (such as iron ore, sinter, etc.) needed to be processed in each process can be calculated reversely, and the energy consumption of raw materials per unit mass of production can be obtained according to the database, literature and actual research data of simapro 7.1 software,
According to the calculation process discussed above, the calculation results of energy consumption of the casting are shown in Table 5.
The carbon emission in the casting molding process is mainly caused by the chemical change of slag forming agent
Considering the scrap rate of castings and the quality of sprue and riser, the comprehensive carbon emission per kilogram of qualified castings is 5.346kgco2e.
2. Analysis of calculation results
2.1 Horizontal analysis and evaluation
The horizontal analysis is mainly to compare the impact of various system energy consumption on system carbon emissions. Through the comparison, it can be found that a specific type of energy consumption is the main factor affecting system carbon emissions, providing the basis for decision makers to formulate energy-saving measures. In the above example, from the horizontal analysis and evaluation chart (as shown in Figure 5), it can be seen that the coke energy consumption of the raw material system is the most important factor affecting the carbon emission of the system, followed by the raw coal energy consumption of the process system.
2.2 Longitudinal evaluation
The purpose of longitudinal analysis is to determine the main subsystem of the casting production system that affects the carbon emission of the system, and optimize each process of the subsystem to achieve the purpose of emission reduction. From the vertical analysis and evaluation chart (as shown in Figure 6), it can be seen that the carbon emission of materials accounts for a large proportion. The carbon emission of materials mainly comes from the raw material system, i.e. the cast iron smelting process system. Therefore, the purpose of emission reduction of the casting production system can be achieved by optimizing the cast iron smelting process and reducing the process energy consumption.
Therefore, in the sand casting production system, the raw material production process is an important link of energy conservation and emission reduction, especially the use of coke in the smelting process, which is an important factor affecting the total carbon emission of the system. At present, the main methods to reduce the use of coke are to improve the chemical composition of coke, improve the combustion rate of coke, etc., so as to achieve the purpose of energy conservation and emission reduction. At the same time, the use of new fuel instead of coke, low cost and high efficiency to reduce carbon emissions, has become an important research direction to achieve low carbon casting.
3 Concluding remarks
Based on the characteristics of sand casting process, the carbon emission estimation and evaluation boundary of the casting production system is established to analyze the factors affecting the carbon emission of the casting production system. In a specific input-output flow, by defining the evaluation boundary of the system, we can clearly point out some influencing factors that may be ignored by decision makers. Taking sand casting as the research object and energy consumption of casting process as the core, the input and output control characteristics of carbon emission sources in the whole production process of casting products are analyzed quantitatively and modeled. The carbon emission evaluation function and calculation method of energy carbon, process carbon and material carbon are established to identify and quantify carbon emission of sand casting production system. By checking the emission function with an example, the quantitative results of carbon emission of typical castings are obtained, and the results are evaluated horizontally, which shows that the main factor affecting the carbon emission of the casting production system is the use of coke; in the longitudinal analysis and evaluation, it is concluded that the carbon emission of the casting production system mainly comes from the carbon emission of the raw material system.
The purpose of this paper is to carry out exploratory research for the realization of low carbon in foundry production. Most of the data are from statistical data, literature, software, and site, and there is no unified value standard and scope. The actual situation of energy consumption in foundry production system is closely related to the production technology level of enterprises. How to obtain reliable calculation basis and obtain carbon emission in line with the actual production system, It will be the focus of the follow-up research work.