Study on the assessment and reduction technology of carbon dioxide from cementing material manufacturing sector

For the purposes of simplifying the calculation task, adjusting production processes in time and solving the inconsistent requirements for carbon emissions, this paper investigates the calculation methods of carbon dioxide emissions from cement production, for example IPCC, WBCSD-CSI, MEE-CBMA, CNIS and BNU. Then a simplification and intuitive method is proposed. Based on the intuitive method, CO2 emission of 21 cement plants in China are calculated and analyzed, of which the error between the calculation results and those obtained by HJ 2519-2012 is less than 0.5%. About the carbon reduction technology in cement industry, there is limited reduction space that rely on energy efficiency improvements and clinker substitution. The technology of alternative fuels still needs to be further expanded. China has operated the first demonstration production line of CCUS technology at the Anhui Baimashan Conch cement plant with a capacity of 20,000 tons/year of industrial-grade liquid CO2 products and 30,000 tons/year of food-grade liquid CO2 products. Alternative raw material technology may be one developing direction to cut carbon emission; only 6.18% of steel slag was added to the raw meal at a 2500t/d production line, CO2 emission from process emissions were reduced by nearly 10%.


Introduction
Cement industry is the fundamental industry sector for the economic construction and production development, which is also a typical resource and energy consumption industry sector. With China's rapid economic and infrastructures growth requirements, 2.33 billion tons of cement was produced by 3427 cement plants, 1712 NSP clinker production lines of China's cement industry in 2019, accounting for nearly 60% of global cement output. According to the cement production processes, a large amount of limestone, coal and power resources are consumed in the preparation of raw meal, clinker calcination and other main processes, which would cause direct or indirect CO2 emission. Cement industry has become the second largest source of CO2 emission in the manufacturing sector, nearly 7% of total CO2 emission. However, carbon dioxide emission reduction application technology in the cement industry lags far behind the advanced level [1].
Facing the climate change and the requirement of carbon management, several organizations have formed CO2 emission calculation and measurement systems of cement manufacturing sector. For example, the Intergovernmental Panel on Climate Change (IPCC) and the World Business Council for Sustainable Development (WBCSD) have promulgated a series of universal measurement methods. The United States and the European Union also have proposed their own measurement standards. Since 2012, China also established series CO2 emission calculation methods for cement plant, providing guidelines for the low carbon cement product certification and national carbon emission trading market in the The carbon dioxide accounting and trading experiences demand that the employees of cement plant learn how to calculate CO2 emissions during cement production, and adjust production processes to reduce or control carbon emissions timely. It may be a big challenge for these [2].
In this paper, to solve those issues, a new, simplification and intuitive method is proposed. The method is a look-up table query method to calculate the CO2 emissions of cement plant, which is simplifying the calculation steps, and making the self-examination more efficient. According the domestic calculation examples and data analysis in China, explore the causes of deviations in the results, and give some suggestions about the future developing direction of cement production emission reductions, such as alternative raw materials, carbon capture, utilization and storage technologies.

Analysis of existing carbon dioxide calculation methods
The following mainly introduces the calculation methods of carbon dioxide emissions from cement production from IPCC, WBCSD-CSI, MEE-CBMA, CNIS and BNU.
The 2006 IPCC Guideline for National Greenhouse Gas Inventories (2006 IPCC Guideline) is provided for greenhouse gas inventory arrangements and management, data gathering, compilation, reporting and estimating anthropogenic emissions by sources and removals by sinks of greenhouse gases, which were published in 2006, and then refined in 2019 [3]. The Greenhouse Gas Protocol for the Cement Industry [4] is provided for a method to calculate the CO2 emission from the cement industry, which was published in 2001, 2005 and 2011.
In China, HJ 2519-2012 Technical requirement for environmental labeling products-Cement was provided for developing and implementing the CO2 statistics, monitoring and assessment methods for  In terms of research scales and calculation emission factor resources, the calculation methods based on the plant data and clinker emission factor is the mainstream approach.

2.2.1.
Major sources of CO2 emissions from cement production. CO2 emissions of cement production are resourced from direct and indirect emissions, which are shown in Table 2 and Figure 1, according to China's cement production processes, equipment, the consumption of raw materials and fuels, electricity consumption, and types of cement products [5][6]. Table 2. Major sources of CO2 emissions from cement production.

Emission resources Explanation
Direct CO2 emission decomposition of carbonate minerals in raw meal CaCO3, MgCO3 material coal consumption in production processes non-fuel carbon burning in raw meal fuel oils consumption in production process diesel alternative fuel consumption waste oil, waste plastic, FRP (glass fiber reinforced plastic), etc. coal and fuel oils consumption in waste co-processing process

Indirect CO2 emission
power consumption in cement production processes cement production process, auxiliary production management power consumption in waste co-processing process pre-treatment, storing and transferring systems outsourcing clinker and fine grinding admixture materials excluding this item when facing the statistics at the national level  Figure 1. CO2 emissions from cement production.

Calculation procedures of intuitive method.
It needs three steps to calculate the CO2 emissions per unit of cement product based on the intuitive method. The first step is to inquire these data, including the CaO and MgO mass fraction in the clinker, the weighted average low heat value of material coal, the material coal consumption per unit of clinker, the ratio of material to cement, the mass percentage of non-fuel carbon in raw meal and other production data [7][8]. And then the second step is to obtain relative parameters, through the look-up table of CO2 emission from the decomposition of carbonate minerals in raw materials, look-up table of CO2 emission from material coal consumption, look-up table of CO2 emission from non-fuel carbon in clinker, look-up table of correction factor of unit comparable quantity emissions of cement clinker production. The last step is to calculate the unit comparable quantity of CO2 emission of cement clinker and cement, through those parameters and equation (1) and equation (2).
(1) Unit comparable quantity of CO2 emission of cement clinker Unit comparable quantity of CO2 emissions of cement clinker should be calculated following the equation (1) with the parameters in Table 3, Figure 2, Table 4, Table 5, equation (14) and equation (15): C ck = (P rc + P bcl + P ro + P ei + P oilcl ) × K ck (1) Where: P rc -CO2 emission from the decomposition of carbonate minerals in raw materials, in units of kg per ton of clinker (kg/tcl); P bcl -CO2 emission from material coal, in units of kg per ton of clinker (kg/tcl); P ro -CO2 emission from non-fuel carbon combustion in raw meal, in units of kg per ton of clinker (kg/tcl); P ei -indirect CO2 emission from power consumption, in units of kg per ton of clinker (kg/tcl); P oilcl -indirect CO2 emission from fuel combustion in various production processes during the statistical period, in units of kg per ton of clinker (kg/tcl).
(2) Unit comparable quantity of CO2 emission of cement Unit comparable quantity of CO2 emissions for cement with different type and strength grade should be calculated following the equation (2) and (3) with the relative parameters of unit comparable quantity of CO2 emission of cement clinker, mass fraction of clinker to cement, mass fraction of outsourcing fine grinding admixture materials and the power consumption of cement grinding: When outsourcing cement clinker and outsourcing fine grinding admixture materials were used for producing the cement, it should be calculated following the equation (2): When there were no outsourcing cement clinker and outsourcing fine grinding admixture materials were used for producing the cement, it should be calculated following the equation (3): (3) CO2 emission from the decomposition of carbonate minerals CO2 emissions from the decomposition of carbonate minerals are composed by CO2 emission from the decomposition of carbonate minerals in raw meal, CO2 emission from the carbonate minerals in the flue gas dust of kiln exhaust stack (kiln outlet) and bypass dust. In actual practice, there is few companies use bypass venting, which can be ignored. Therefore they can be calculated by the equation (4) Since the measured value of Ue generally fluctuates around the default value, 0.15 kg/t can be used as the default value of Ue in this method. According to the value range of Cc and Cm and the Matlab software, the full array of CO2 emission from the decomposition of carbonate minerals are obtained as shown in Table 3.  (4) CO2 emission from the combustion of material coals CO2 emission from the combustion of material coals in production processes should be calculated following the equation (5): P bcl = P cl ×Q nck ×J×F b 29.307×1000 (5) Where: Pcl-comprehensive coal consumption per unit of clinker in the statistical period, in t/t; Qnck-the weighted average low heat value of material coal for various batches into plant in the statistical period, in kcal/kg; J-the mechanical equivalent of heat, which is 4.186 kJ/kcal; Fb-CO2 emission factor of standard coal, in t/tce; it is determined uniformly by the state as 2.75 t/tce; 29.307-the calorific value of standard coal, in MJ/kgce. Though Matlab software and all relative parameters, Figure 2 can be obtained by simplifying the equation and taking the contour. And CO2 emission from the combustion of material coals per ton of clinker can be found in the Figure 2, through the comprehensive coal consumption per unit of clinker and the weighted average low heat value of material coal.  Figure 2. Look-up chart of CO2 emission from material coal consumption(Pbci) (5) CO2 emission from the burning non-fuel carbon of raw meal CO2 emission from the burning non-fuel carbon of raw meal should be calculated following the equation (6): P ro = r a × R o × 44 12 × 1000 (6) Where: ra-the ratio of material to cement; if missing statistical data, 1.52 can be used as the default value; Ro-the non-fuel carbon fraction of raw meal, %; if missing statistical data, 0.1%～0.3% (in dry state) can be used as the default value. When coal gangue and high carbon fly ash are used in the raw meal preparation, it is taken as 0.3%; otherwise it is taken as 0.1%; 44/12-molecular weight conversion between CO2 and C; In actual calculation process, the actual value of ra is generally between 1.5~1.6. Table 4 is obtained by doing a full permutation of P ro . Indirect CO2 emission from power consumption includes all the CO2 emission from power consumption excluding cement grinding, shipping and packaging, which should be calculated following the equation (7): Where: E1-power consumption in mining and mineral transportation, kW· h E2-power consumption in raw meal preparation, kW· h E3-power consumption in clinker calcination, kW· h E5-power consumption in auxiliary production management, kW· h Er-net power generation by waste heat, kW· h Fe-CO2 emission factor of electricity, in kg/kW· h; it is determined uniformly by the state as 0.86 kg/kW· h.
(7) indirect CO2 emission from fuel consumption Indirect CO2 emission from fuel consumption includes all the CO2 emission from the fuels of inplant vehicle transportation and kiln startup ignition, which should be calculated following the equation (8): Where: Oildi-the amount of diesel oil used in each production process during the statistic period, in t; Oilgl-the amount of gasoline used in each production process during the statistical period, in t; Q noldi -low calorific value of diesel oil, as 43 MJ/kg; Q nolgl -low calorific value of gasoline, as 44.3 MJ/kg; Fdi-CO2 emission factor of diesel oil, as 0.0741 kg/MJ; Fgl-CO2 emission factor of gasoline, as 0.0700 kg/MJ. In the self-examination or rough calculation of the cement enterprise, since the gasoline consumption in the plant is generally small, the gasoline consumption can be integrated into the uniform calculation of the diesel, thereby for reducing the calculation content, of which the error value is below 1 ‰.
(8) correction factor of unit comparable quantity CO2 emissions of cement clinker (Kck) It is necessary to obtain the unit comparable quantity of CO2 emission of cement clinker when carrying the self-examination or rough calculation of the cement enterprise, then the correction coefficient (Kck) is introduced, which is mainly the conversion of strength and altitude for the cement enterprise. Through the Matlab software, the total arrangement of Kck is programmed as shown in Table  5. Table 5. Look-up table of correction factor of unit comparable quantity CO2 emission of cement clinker, Kck.

Case study of assessment of carbon dioxide in China cement plants
Though the above intuitive method, CO2 emission of 21 cement manufacturers in China are calculated and shown in Table 6, which include direct CO2 emissions, indirect CO2 emissions, and CO2 emission from the carbonate decomposition, CO2 emission from coals. The error between the calculation result of this intuitive method and the result obtained by using the method of HJ 2519-2012 Technical requirement for environmental labeling products-Cement is less than 0.5%. Regarding to the Wf1 cement plant, it takes use of the sandstone, steel slag and fly ash for raw meal preparation, with the clinker output of 1.8429 million tons, dusts of 0 ton from kiln outlet and bypass system. And it consumes sandstone of 120,000 tons, steel slag of 150,000 tons (100,000 tons for cement grinding), fly ash of 50,000 tons (30,000 tons for cement grinding). The annual weighted CaO and MgO contents of clinker and alternative raw materials are shown in Table 7. Therefore, non-carbonate raw materials in the case include steel slag of 50,000 tons and fly ash of 20,000 tons. CO2 emission from the raw material decomposition is calculated and shown in Table 8, starting from the perspective of process characteristics and material characteristics.  Replacing traditional carbonates with alternative raw materials such as industrial waste could significantly reduce CO2 emissions. Carbonate decomposition generates more than 50% of CO2 emission in cement production. Taking one above 2500t/d production line as an example, only 6.18% of steel slag was added to the raw meal, but CO2 emission from process emissions were reduced by nearly 10%.

Conclusions
The initial problems and targets have been realized through the intuitive method, which covered simplifying the calculation steps, efficient CO2 emission self-examination and CO2 emission from raw material carbonate decomposition. With the application of the method, CO2 emission of 21 cement plants in China are assessed and analyzed. Carbon sources throughout the life cycle of cement production give suggestion about the future developing direction of cement production, such as alternative raw materials, carbon capture, utilization and storage technologies.