Accounting methods and influencing factors of carbon Emission in Aluminum Alloy Foundry Enterprises

In September 2020, China promised at the United Nations General Assembly that China's CO2 emissions would reach its peak by 2030 and strive to achieve carbon neutrality by 2060. Carbon emission is an abbreviation about greenhouse gas emissions, including carbon dioxide, CH emissions, nitrous oxide, hydrofluorocarbons, perfluorocarbons, SF emissions and so on. The main gas is carbon dioxide. In order to simplify data statistics and analysis, equate "carbon emissions" with "carbon dioxide emissions" and ignore the analysis and calculation of other greenhouse gases.

The production capacity of aluminum alloy casting and electrolytic aluminum in China has made rapid development in the past 20 years, but it is also faced with problems such as outdated equipment, low energy efficiency and high carbon emissions, which has caused great pressure on the realization of the goal of carbon neutralization. With the continuous promotion of the national carbon emission policy to the downstream industry, aluminum alloy casting, an industry with high energy consumption and high emissions, is facing greater pressure and challenges in emission reduction. Therefore, it is of great practical significance to study the accounting methods and influencing factors of carbon emission of aluminum alloy casting in China under the constraints of carbon emissions.

1 Research methods and data sources

1.1 Research methods

Because it is difficult to collect accurate data of carbon emissions in the production processes of bauxite mining, alumina roasting, electrolytic aluminum, aluminum alloy smelting, aluminum alloy casting, heat treatment, machining, coating, etc., therefore, the author analyzes and explains the data according to the estimated value of energy consumption. The carbon emissions of natural gas combustion and power consumption are calculated and analyzed according to the China Energy Statistical Yearbook and the guidelines for Accounting methods and reporting of greenhouse Gas emissions from other Nonferrous Metals smelting and Calendering Enterprises. The calculation formulas are as follows:

CO2 is the total carbon emission, Ei is the energy consumption, EFi is the carbon emission factor, NCVi is the average low calorific value, NCEi is the carbon content per unit calorific value, and COFi is the carbon oxidation rate. The reference standards for the factors needed in the calculation come from the National greenhouse Gas Emission inventory Guide of IPCC 2006.

1.2 data sources

According to the statistical analysis of the China Power Union, in 2018, the average coal consumption of thermal power units was 308 g / kW ·h (net efficiency was 39.9%), and the CO2 emission intensity of thermal power plants was about 841 g / kW ·h, that is, the CO2 emission factor was 0.841 0 kg CO2/ kW ·h; According to the guidance on Energy Saving and Emission reduction in Nonferrous Metals Industry, the benchmark index of MIIT for direct current consumption per ton of electrolytic aluminum is 12500 kW ·h, that is, for every ton of electrolytic aluminum ingots prepared by thermal power, the CO2 emission is 10.51t; the aluminum ingots produced by electrolytic aluminum ingots using clean electric energy such as water and light are regarded as clean aluminum ingots, and the carbon emissions produced by clean electric energy are regarded as zero in carbon emission accounting. Carbon emissions from clean aluminum ingots mainly come from carbon emissions from bauxite mining, alumina roasting, electrolytic aluminum operation, graphite anode and other thermal energy.

Table 1 shows the carbon emission accounting factors in the process of the study. According to the calculation of the data in Table 1, the total emission of CO2 per ton of clean aluminum ingots (all using clean electricity) is 6.05t (2.9mm 3.1m. 05), while that of electrolytic aluminum is thermal power, and the total emission of CO2 per ton of aluminum in the production process is 16.56t (10.51t 6.05).

2 Enterprise research and emission reduction measures

2.1 Enterprise Research

Take an aluminum alloy casting hub factory in Zhejiang as an example, it produced 5 million wheels in 2020; aluminum ingots consumed 81300 tons, of which clean electricity accounted for 26% of electrolytic aluminum production; total electricity consumption was 82.86 million kW ·h, of which clean electricity accounted for 16.35% and natural gas consumed 16.12 million m3.

As can be seen from Table 2, the aluminum alloy wheel casting enterprise has a total of 1.217 million tons of CO2 emissions in 2020, of which the preparation of aluminum ingots indirectly emits CO2 1.124 million tons, the carbon emissions of electric power consumption is 58000 tons, and the carbon emissions of natural gas consumption are 35000 tons, accounting for 92.35%, 4.77% and 2.88%, respectively. The carbon emissions from the preparation of aluminum ingots include the total carbon emissions from bauxite mining to electrolytic aluminum completion.

With the promotion of the national carbon peak and carbon neutralization strategy, the prices of clean electricity and clean aluminum ingots will increase year by year. Before 2021, clean electricity and clean aluminum ingots basically maintained the same pricing mechanism, but starting from 2021, the domestic prices of clean electricity and clean aluminum ingots gradually increased under the influence of the national carbon neutralization policy. there is a price difference equivalent to non-clean electricity and non-clean aluminum ingots; such as the introduction of a new policy in some areas of Zhejiang Province: increasing the proportion of clean electricity (purchasing green electricity) by 10% will increase the cost of electricity by 6.9%; At the same time, the price of hydropower and aluminum produced by clean aluminum ingots in various places, especially in Yunnan, is 1.6% higher than the average price of the industry (comparison of bulk purchase prices of enterprises).

If the energy structure and production process of Chinese aluminum ingot production enterprises reach the level of Europe and the United States, that is, clean electricity accounts for 80%, electrolytic aluminum adopts new technologies and processes such as environmentally friendly inert anodes, as shown in Table 3, CO2 emissions from aluminum alloy foundry plants with the same production capacity will be reduced to 613000 t, a reduction of 49.63%; at the same time, the corresponding electricity procurement costs and aluminum ingot procurement costs will also increase.

As can be seen from Table 3, after increasing the proportion of clean electricity and adopting the new process of environmental protection electrolytic aluminum, the carbon emissions are significantly reduced, but if we further reduce carbon emissions, we need to make a major breakthrough in the technology and process of clean electricity instead of natural gas energy.

2.2 measures to reduce carbon emissions in aluminum alloy casting enterprises

For aluminum alloy casting enterprises, the following measures can be taken to reduce carbon emissions.

(1) to increase the procurement proportion of clean primary aluminum ingots and recycled aluminum.

According to the IAI data of the International Aluminum Association, the carbon emissions of China's electrolytic aluminum industry are higher than the global average, reaching as high as 11.2t (per tonne of aluminum), because domestic electricity is heavily dependent on thermal power (that is, coal power, accounting for about 85 per cent), and the proportion of hydropower is only slightly more than 10 per cent, far lower than the global hydropower share of nearly 25 per cent. As more and more aluminum stocks in use enter the scrap period, the production of aluminum scrap will continue to increase, and the production of recycled aluminum can exceed that of primary aluminum around 2040. As the energy consumption of recycled aluminum production is only 4.9% of that of primary aluminum, and the carbon emission is only 4.2% of that of primary aluminum, as primary aluminum is gradually replaced by recycled aluminum, the peak of primary aluminum production before 2030 will promote the realization of carbon emissions peak.

Due to the different power supply structure and process technology of electrolytic aluminum or primary aluminum production in different countries and regions, the carbon emissions produced by aluminum ingot production are also quite different; of course, the power supply structure is the most important factor. At present, hydropower is the cleanest source of energy, and the carbon emissions of hydropower and aluminum enterprises are the lowest, while those of coal, electricity and aluminum are the highest. For example, some hydropower and aluminum enterprises in Yunnan Province account for more than 90% of their power supply structure.

In the procurement of aluminum ingots, aluminum alloy casting enterprises should gradually increase the proportion of hydropower in primary aluminum ingots, or study the feasibility of using recycled aluminum instead of primary aluminum, and gradually increase the proportion of recycled aluminum.

(2) increase the proportion of clean energy electricity.

At present, among the most important ways of renewable energy, hydropower, wind power and biomass power generation are only suitable for centralized power generation and are not suitable for enterprises. Nuclear power is green energy from the perspective of carbon emissions, but it is not renewable energy, and it is only suitable for large-scale centralized power generation with high technical requirements; by contrast, solar energy is the only way to generate electricity that can be miniaturized at present. there are no geographical restrictions on renewable energy, and China has the core advantages of technology, manufacturing and application scale in photovoltaic power generation. Among them, the photovoltaic power generation project at the top of the plant can basically recover the input cost in 5-8 years, and the continuous benefit of the project can be as long as 25 years. Photovoltaic power generation at the top of the plant has become the main source of clean power for aluminum alloy casting plants.

(3) improve energy efficiency

In the molten aluminum smelting process of an aluminum alloy foundry, the carbon emissions from the combustion of natural gas usually account for 80% of the direct carbon emissions of the whole plant. Compared with the indirect carbon emissions generated by purchased aluminum ingots and electric power, the direct carbon emissions generated in the foundry production process are affected by the enterprise's own equipment energy efficiency, advanced production process, production management level and other factors.

According to the various energy saving and emission reduction measures carried out by the research factory and their implementation results, through the intelligent and energy-saving transformation of the old melting furnace, such as the application of high-quality thermal insulation materials, heat storage burners and intelligent temperature control system, the energy efficiency of the melting furnace and static furnace has increased by 16.5%. Based on the annual natural gas consumption of 9.58 million m3, the annual CO2 emission can be reduced by 3414 t, and the annual natural gas cost can be saved by more than 4 million yuan. The payback period of equipment investment is only 1.8 years.

(4) waste heat utilization of smelting and heat treatment.

The high temperature flue gas from melting furnace, static furnace and heat treatment furnace in aluminum alloy casting plant has high waste heat utilization value. through the waste heat recovery device, the Scald produced can be supplied to machining cleaning line, painting cleaning line, employee bathing and other equipment to reduce natural gas consumption and achieve energy saving and emission reduction. Taking the investigated factory as an example, through the utilization of the waste heat of melting furnace and heat treatment furnace, the annual natural gas consumption can be reduced by 1.025 million m3, the CO2 emission can be reduced by 2 214 t, the annual natural gas cost can be saved by more than 2.5 million yuan, and the payback period of equipment investment is only 2.4 years.

(5) improve the level of casting process design and the qualified rate of production.

The level of casting process design and the qualified rate of production not only determine the production cost and profit level of the enterprise, but also have a great influence on the direct carbon emission of castings. According to the investigation factory data, by improving the casting mold design level to improve the casting process yield and reduce casting shrinkage, air leakage and other bad phenomena, reduce the scrap rate of casting blank. At the same time, measures such as improving the qualified rate of painting can reduce the direct carbon emissions of the factory by 9%, which not only achieves the annual target of energy saving and emission reduction, but also reduces the production cost.

3 conclusions and suggestions

In the past, most of the carbon emission accounting studies focused on the construction of life-cycle carbon emission accounting methods in iron and steel, power generation, cement, automobile manufacturing and other industries, but there were few studies on carbon emissions in subdivided industries, especially in aluminum alloy casting. Based on the detailed investigation of the carbon emission data of domestic aluminum alloy hub production plants, the author explores the accounting methods and influencing factors of carbon emissions in aluminum alloy casting industry, and puts forward policy suggestions to achieve the goal of carbon neutralization in aluminum alloy casting enterprises.

(1) through the analysis of the carbon emission data of the survey factory, it is found that the indirect carbon emissions of aluminum alloy foundry, such as CO2 emissions generated by aluminum ingots and electricity, account for 94.29%; the direct carbon emissions mainly refer to the CO2 emissions from natural gas combustion, accounting for 5.71%.

(2) through the measures of smelting equipment transformation and factory waste heat utilization, it is estimated that the CO2 emission will be reduced by 28000 t in the next 5 years, and the total cost can be reduced by more than 18 million yuan, but with the increase of aluminum alloy foundry enterprises' demand for clean electricity and clean aluminum ingots or recycled aluminum ingots, it is bound to drive up their prices, resulting in an increase in clean power and clean aluminum ingots procurement costs.

(3) the main factors affecting the CO2 emission of aluminum alloy foundry include the proportion of clean electricity during the preparation of aluminum ingot and its electrolytic aluminum technology, the proportion of clean electricity in aluminum alloy foundry, the energy utilization efficiency of equipment and the waste heat utilization rate of the plant, etc. Among them, increasing the proportion of clean energy electricity is the most direct and effective way to reduce carbon emissions, and using clean electricity instead of natural gas energy is the only way for aluminum alloy casting enterprises to achieve the goal of carbon neutralization.