1、 What is carbon accounting?

Since the adoption of the Kyoto Protocol in 1997, countries all over the world have carried out a series of emission reduction measures to deal with the climate change brought by industrialization. However, emission control entities such as different countries, regions and enterprises need to rely on scientific data to clarify carbon reduction objectives and measure carbon reduction results. Carbon accounting is a measure to measure the direct and indirect emissions of carbon dioxide and its equivalent gases from industrial activities to the earth's biosphere. It can be seen that from the perspective of accounting objects, carbon accounting needs to include at least the following two conditions: one is to delimit the gases causing the greenhouse effect, and the other is to determine the main body of industrial activities.

Greenhouse gases are natural and man-made gaseous components that absorb and re emit infrared radiation in the atmosphere, including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCS), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3). Because different gases have different degrees of impact on the greenhouse effect, the United Nations Intergovernmental Panel on climate change (IPCC) proposed the concept of carbon dioxide equivalent (CO2e) to uniformly measure the impact of these gas emissions on the environment. Based on the global warming potential (GWP), we can see the impact of different gases on the greenhouse effect relative to carbon dioxide.

In addition, only for energy activities and industrial production processes, according to the guidelines for the preparation of provincial greenhouse gas inventories, HFCS, PFCs and SF6 mainly involve a few industrial production processes such as aluminum and magnesium, while N2O has long been included in the scope of air pollution monitoring. Therefore, the main objects of carbon accounting for most enterprises are CO2 and CH4. According to the 2017 China greenhouse gas bulletin, carbon dioxide (CO2) and methane (CH4) are the main and secondary long-lived greenhouse gases affecting the earth's radiation balance, and their contribution rates to the total radiation forcing caused by the increase of the concentration of all long-lived greenhouse gases are about 66% and 17% respectively.

Source: Federal greenhouse gas

Accounting and Reporting Guidance

From the perspective of the main body of industrial activities, according to the IPCC guidelines for national greenhouse gas emission inventories and the guidelines for the preparation of provincial greenhouse gas inventories, carbon accounting mainly covers five activities: energy activities, industrial production, agricultural production, forestry and land use change, and waste disposal.

For the above accounting subjects, carbon accounting can generate different types of carbon accounting results according to data sources, measurement methods, data forms, data quality, measurement regions, time ranges and other factors.

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2、 What are the methods of carbon accounting?

The main forms of carbon accounting can be divided into measurement based and calculation based methods. Specifically, from the existing accounting methods of greenhouse gas emissions, it can be summarized into three kinds: emission factor method, mass balance method and actual measurement method. At present, the greenhouse gas quantification methods adopted in the 24 guidelines published by the national development and Reform Commission only include the emission factor method and the mass balance method. However, the national measures for the administration of Carbon Emission Trading (Trial) issued by the Ministry of ecology and environment in December 2020 clearly pointed out that key emission units should give priority to the measurement of low calorific value and carbon content of fossil fuels.

（1） Emission factor method (based on calculation)

Emission factor method is the most widely used and widely used carbon accounting method. According to the basic equation of carbon accounting provided by IPCC:

Greenhouse gas (GHG) emissions = activity data (AD) × Emission factor (EF)

Among them, ad is the activity amount of production or consumption activities leading to greenhouse gas emissions, such as the consumption of each fossil fuel, the consumption of limestone raw materials, the net purchased electricity, the net purchased steam, etc; EF is the coefficient corresponding to the activity level data, including carbon content per unit calorific value or element carbon content, oxidation rate, etc., which represents the greenhouse gas emission coefficient per unit production or consumption activity. EF can be directly calculated by using the known data (i.e. default value) provided by IPCC, US Environmental Protection Agency and European environmental agency, or based on representative measurement data. China has set national parameters based on the actual situation. For example, Appendix II of the guidelines for accounting methods and reporting of greenhouse gas emissions by enterprises in other industries (for Trial Implementation) provides the default value data of common fossil fuel characteristic parameters.

This method is applicable to the macro accounting level of countries, provinces and cities, and can roughly control the overall situation of specific regions. However, in practical work, due to regional differences in energy quality and unit combustion efficiency, various energy consumption statistics and carbon emission factor measurement are prone to large deviations, which has become the main source of error in carbon emission accounting results.

（2） Mass balance method (based on calculation)

The mass balance method can calculate the share of new chemicals consumed to meet the capacity of new equipment or replace and remove gas according to the new chemicals and equipment used for national production and life every year. For carbon dioxide, under the carbon mass balance method, the carbon emission is obtained by subtracting the carbon output of non carbon dioxide from the input carbon content:

Carbon dioxide (CO2) emission = (raw material input) × Carbon content of raw materials - product output × Product carbon content - Waste Output × Waste (carbon content) × 44/12

Where, is the conversion coefficient of carbon to CO2 (i.e. the relative atomic mass of CO2 / C). The carbon mass balance method based on specific facilities and process flow is used to calculate the emissions, which can reflect the actual emissions in the place where carbon emissions occur. It can not only distinguish the differences between various facilities, but also distinguish the differences between single and partial equipment. Especially when the equipment is constantly updated in the current year, this method is more simple. Generally speaking, the main accounting method for enterprise carbon emissions is the emission factor method, but in the industrial production process (such as desulfurization process emissions, chemical production enterprise process emissions and other non fossil fuel combustion processes), the carbon balance method can be selected according to the situation.

（3） Measurement method (based on measurement)

The measurement method is based on the measured basic data of emission sources, and the relevant carbon emissions are summarized. Here, there are two measurement methods: on-site measurement and off-site measurement.

The on-site measurement is generally to install the carbon emission monitoring module in the flue gas emission continuous monitoring system (CEMS), and directly measure its emission through continuous monitoring of concentration and flow rate; Off site measurement is to collect samples and send them to relevant monitoring departments for quantitative analysis using special testing equipment and technology. Compared with the two, the accuracy of on-site measurement is significantly higher than that of off-site measurement due to the problems of adsorption reflection and dissociation of sampled gas in off-site measurement.

In 2011, the US Environmental Protection Agency (EPA) mandated the installation of all carbon dioxide equivalent emission monitoring systems online, which exceeded the maximum of 50000 tons in 2011.

The European Commission has launched the EU carbon emissions trading system since 2005 and officially carried out the monitoring of CO2 emissions. However, only 155 emission units (accounting for 1.5%) in 23 countries have used CEMS (continuous monitoring system), mainly Germany, Czech Republic and France.

CEMS has been basically installed in thermal power plants in China, which has the basis of using CEMS to monitor CO2 emissions. On May 27, China's first accurate carbon emission measurement system for the power industry was launched in Jiangsu. It took the lead in applying the measured method to conduct real-time online monitoring and accounting of carbon emissions in China. It is expected to be popularized throughout the country soon.