Study of Energy Use and CO₂ Emissions in the Manufacturing of Clinker and Cement

Abstract

Cement is one of the most widely used materials in construction. This study presents a process-wise evaluation of energy use and CO₂ emissions for clinker, Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC) using the principles of life-cycle assessment. Two cement plants located in India are considered as typical case studies. The gate-to-gate system boundary condition is considered. The energy use for clinker is found to be 3990 and 3626 MJ/ton for case studies 1 and 2, respectively. The associated CO₂ emissions for clinker are 849 and 868 kg CO₂/ton. The energy use for OPC is 4015 and 3821 MJ/ton for case studies 1 and 2, respectively. The related CO₂ emissions are 802 and 855 kg CO₂/ton. The energy use for PPC is 3077 and 2733 MJ/ton for case studies 1 and 2, respectively. The associated CO₂ emissions are 606 and 595 kg CO₂/ton. It is observed that the energy use and CO₂ emission of PPC are at least 20% less compared to OPC. The results are compared with five geographical regions across the world.

Introduction

Cement is one of the most widely used materials in the construction industry. The production and consumption of cement has increased with time globally. According to the United States Geological Survey (USGS), the global cement production was 1370 Million tons (MT) in the year 1994 [21]. This has increased to 4100 MT in 2017 which is more than three times [24]. Major cement producing countries in the year 2014 are China (2492 MT), India (275 MT), and USA (83 MT). These countries represent 59.6%, 6.6% and 2% of the world production, respectively. Imbabi et al. [9] reports that the world cement production is expected to be as high as 5500 MT by 2050. China and India are expected to be the major contributors towards global cement production.

India is the second largest cement producer in the world [2]. The Department of Industrial Policy & Promotion of the Ministry of Commerce and Industry reports that India’s cement production reached 283 MT during 2015–2016 [16]. Morrow et al. [18] predicts that India’s cement industry is anticipated to produce between 646 and 742 MT cement per year by 2030. Fonta et al. [6] projects that Indian cement production could be 780 to 1360 MT by 2050. India is an emerging nation and significant growth is expected in the infrastructure development sector. The annual per-capita cement production of India is 178 kg, which is about 17% of the per-capita cement production of China [20]. The Indian cement industry is expected to post high growth in cement production in the coming decades.

The cement industry is one of the highly energy-intensive sectors in India with an annual energy consumption of 15.01 million ton oil equivalent [17]. The annual green house gas emissions (GHG) of the cement industry are about 129 MT in the year 2007, which is 6% of the total GHG emission and 32% of the total CO₂ emissions from the industrial sector in India [15]. The International Energy Agency (IEA) baseline scenario predicts that the energy consumption of the Indian cement industry would be between 42 and 48 MTOE by 2050 [22]. In the absence of appropriate technological and policy measures, CO₂ emissions from the Indian cement industry are expected to be between 488 MT CO₂ (low demand) and 835 MT CO₂ (high demand) by 2050 [6]. These statistics highlights the need for a detailed evaluation and monitoring of energy use and emissions from cement production processes in India.

Table 1 presents an overview of energy use for cement production across five geographical regions in the world. Table 2 presents emissions generated due to cement production. The total energy use and emissions along with the breakup by key components are presented in Tables 1 and 2. Madlool et al. [14] report that the thermal energy used for cement production varies from 3.5 to 4.5 GJ/ton of cement and the electric energy varies from 92 to 141 kWh/ton across different countries in the world. It is reported that CO₂ emissions of cement production in Australia are about 820 kg CO₂ equivalent/ton of cement [5]. According to the Cement Sustainability Initiative’s (CSI) Getting Number Right (GNR) project, the global average of gross CO₂ emissions is 842 kg CO₂/ton of clinker.

Table 1 Energy use for clinker, OPC and PPC

Table 2 CO₂ emission for clinker, OPC and PPC

Several studies have been carried out on the energy use and CO₂ emissions related to cement production [1, 4, 6, 11, 19]. Praseeda et al. [19] reported that the energy use for cement production is 2.91 and 4.32 MJ/kg of cement based on two cement plants. This study considers the energy use for raw materials (including limestone), transportation of raw materials, mixing and grinding of raw materials, clinker production, grinding of clinker and packing to determine the energy use. Based on life-cycle analysis using input–output transaction tables, the energy use for cement production is found to be 4.67–8.05 MJ/kg [19]. Fonta et al. [6] state that the Indian cement industry’s average CO₂ emissions have reduced from 1.12 ton CO₂/ton of cement in 1996 to 0.719 ton CO₂/ton of cement in 2010. The gross CO₂ emissions corresponding to the Indian cement industry is about 828 kg CO₂/ton of clinker [4] Studies on energy balancing, specific energy consumption and technologies related to clinkerization processes report limited information on energy use for clinker and cement. Consequentially, there is a need for studies on energy use and CO₂ emissions based on reliable life-cycle inventories in India.

Life-cycle assessment (LCA) is a globally accepted scientific method used for assessing the environmental sustainability of a product or a service. It connects the life-cycle inventory with the environmental impacts. Development of life-cycle inventory is the first step in the sustainability assessment. Many countries have reliable life-cycle inventory databases which are recorded and reported by government and non-governmental organizations. There are also databases available that facilitate compilation of LCI data of products across different countries. These databases enable quick analysis and understanding of energy consumption, CO₂ emissions, or related environmental issues [8]. There is lack of LCI databases for building materials in India including cement. Lack of availability of reliable LCI data is the main challenge in conducting LCA studies for cement production in India. Currently, the eco-invent in collaboration with the Confederation of Indian industry’s (CII) GreenCo is working towards developing LCI datasets for industrial products in India including cement. However, these datasets are not yet available for public use.

Several studies are reported on the energy use and emissions of cement production in India. However, earlier studies do not address the following aspects: (1) process-wise breakup of energy use and emissions for clinker and cement production are not reported in a comprehensive manner; (2) lack of clarity in presenting the system boundary conditions which cause variability in results; (3) several studies carried out earlier present the results for a generic cement; (4) several Indian cement manufacturers participated in the GNR project initiated by the Cement Sustainability Initiative and compiled the data needed for energy use and CO₂ emissions calculation. However, the results based on this initiative are given in the form of performance indicators which do not consider all processes from mining to packaging; and (5) earlier studies do not present the inventory data in a format as prescribed by the ISO 14044:2006 [13]. Further, the life-cycle inventory data for clinker and cement production using the concepts of LCA method is not available for public use. This study aims to follow a process-based life-cycle assessment approach and determine the energy use and emissions of clinker and cement production by considering the entire manufacturing phase. The objectives of this study are to compile the life-cycle inventory data as per the ISO 14044:2006 [13] standard and use the inventory data to estimate the energy use and CO₂ emissions associated with the production of clinker, Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC) using gate-to-gate system boundary.

Methodology and Case Studies

This study is based on the data collected from two cement plants located in the State of Tamil Nadu in India. Both cement plants are located in the Ariyalur district of Tamil Nadu. The first plant (case study 1) has a capacity of 1.6 million tons per annum (MTPA). The second plant (case study 2) has a capacity of 1.5 MTPA. These cement plants use dry processing technique for clinker production and are located near to the limestone mines. A template developed based on ISO 14040 and ISO 14044 guidelines is used for the inventory calculation [12, 13]. Life-cycle assessment consists of four phases namely goal and scope definition, life-cycle inventory, life-cycle impact assessment and interpretation.

Goal and Scope

The objective is to quantify the energy use and CO₂ emissions of production processes of clinker, OPC, and PPC. Process system considered in this study is an integrated cement plant with a functional unit of 1 ton output. Gate-to-gate system boundary is considered. The scope includes processes from limestone extraction till clinkerization for clinker production and remaining processes till packing for cement production. The processes of limestone mining and transportation of limestone to cement plant are also considered as a part of the system boundary so as to model all processes under the control of a cement plant. An expected list of inventory data corresponding to production processes are listed to identify the required data to be collected.

Data quality parameters satisfied are as follows: (1) time: one-year temporal coverage (2014–2015), (2) location: cement plant is geographically situated near limestone belts, (3) technology: dry processing technology with preheater and precalciner for clinker manufacturing and ball mill for cement grinding (4) completeness: data related to energy and CO₂ calculation are almost fully collected, (5) consistency: inventory data, emission factors and the method of calculation are consistent, (6) ability to reproduce the results: data have reproducibility to regional average, (7) source of data: majority are actual data measured at the cement plant and few from databases, and (8) uncertainty and redundancy: list the multiple sources of data and prioritize based on the dependability of the source file.

No allocation is required for a single product like clinker to determine the energy use and emissions, whereas mass allocation is followed for OPC and PPC. Most of the electricity used in the cement factory are produced within the factory using the captive power plant. The energy use and CO₂ emissions associated with electricity use are considered by tracking the amount of raw materials used in the internal captive power plant such as lignite, petcoke and coal.

Life-Cycle Inventory (LCI)

First, the data collection is planned through the development of a series of process flow charts describing the manufacturing processes and listing out the required data [7]. The second step involves data collection, formatting and compilation. Data validation is taken into account in the third step. Inventory data are normalized with respect to functional unit in the fourth step. Fifth step involves aggregation of results based on data type. The final step relates to redefinition of system boundary conditions as the study progresses depending upon the lack of availability of data. The ratio of clinker, limestone and gypsum in OPC for case studies 1 and 2 is 91:5:4 and 95:3:2, respectively. Similarly, the ratio of clinker, fly ash and gypsum in PPC for case studies 1 and 2 is 68:28:4 and 65:33:2, respectively.

Energy Use and CO₂ Emissions Calculation

The life-cycle inventory data are converted into energy use and CO₂ emissions using the energy and CO₂ emission factors. Most of these factors are gathered from cement plants based on the laboratory experiments carried out at these plants. Calculation of energy use is based upon the amount of fuels used for clinkerization such as coal, lignite and petcoke as well as the electricity used within the factory. To determine CO₂ emissions, the decarbonization of carbonates present in the raw meal is taken into account in addition to the amount of fuels and electricity used.

Energy use due to the use of fuels is determined using factors from the following sources (in the order of priority): factors provided by the cement plant, factors based on the bomb calorimetry analysis of fuel samples, energy factors provided by the U.S. Environmental Protection Agency [23] and the guidelines presented by the Intergovernmental Panel for Climate Change [10]. Sources of CO₂ emission factors used in the order of priority are as follows: laboratory tests carried out at the cement plant, CHNS (carbon, hydrogen, nitrogen and sulphur) tests of fuel samples, emission factors of the U.S. EPA [23], CSI Protocol 2013 [3] and the factors of the national greenhouse gas inventory by the Intergovernmental Panel for Climate Change [10].

The factors for electricity use are derived from the inventory data of the internal thermal power plant (TPP) and factors from the above sources. The factor for calculating emissions from raw meal is based on the stoichiometric ratio of CO₂ with CaO and MgO content of raw meal. The calculation follows the principle followed in the ‘calcb2’ method of the Cement Sustainability Initiative [3]. A set of data which contributes to energy and CO₂ are selected from the life-cycle inventory during data assigning and are used for determining energy use and emissions. The factors selected or estimated corresponding to each of the assigned data are used for converting the inventory data into energy and CO₂ emissions. The classified life-cycle inventory data and the calculation details of energy use and CO₂ emissions of clinker, OPC and PPC are presented in Tables 7, 8, 9, 10, 11 and 12 of “Appendix”.

Results and Discussion

Energy Use Associated with Clinker

Table 3 presents the energy use for clinker production for case studies 1 and 2. The energy use needed for each process by data type is summarized. The energy used for clinker production is found to be 3990 MJ/ton (case study 1) and 3626 MJ/ton (case study 2). The thermal energy used for clinkerization and the electricity use forms the two major components of the total energy used. These two components represent 3882 MJ/ton for case study 1 and 3585 MJ/ton for case study 2. The corresponding values for five regions across the world (region 1 to region 5) stated in Table 1 are 3095, 3556, 3139, 2771 and 3067 MJ/ton, respectively. It is observed that the energy used for clinkerization and electricity use for case studies 1 and 2 are higher than the other regions reported in Table 1.

Table 3 Energy use associated with Clinker

The thermal energy used in clinkerization for case studies 1 and 2 is 3080 MJ and 2918 MJ, respectively. The Ministry of Power of the Government of India has reported that the thermal energy consumption for clinkerization is around 658–1074 kcal/kg of clinker, which is around 2753–4494 MJ/ton of clinker [17]. Thus, the thermal energy values of case studies 1 and 2 are in the lower range compared to the literature because the plants in the case studies considered are modern and efficient in terms of heat recovery. The electricity consumption for case studies 1 and 2 is found higher than the values reported in Table 1.

CO₂ Emissions Associated with Clinker

Table 4 presents the summary of CO₂ emissions associated with clinker production. Total CO₂ emissions for clinker production are found to be 849 kg CO₂/ton for case study 1 and 868 kg CO₂/ton for case study 2. Major source of emissions is the direct CO₂ emissions from fuel and raw materials during clinkerization, which is determined as 776 kg CO₂/ton and 811 kg CO₂/ton for case studies 1 and 2, respectively. These values are less than the direct CO₂ emissions of many regions presented in Table 2.

Table 4 CO₂ associated with clinker

Energy and CO₂ Associated with OPC and PPC

Table 5 presents the energy use for OPC and PPC based on case studies 1 and 2. It is observed that the major contributors of energy use are clinker and electricity used for grinding and packing. In case of OPC, the energy used for major contributors (clinker and electricity use) is found to be higher compared to several regions reported in Table 1; however, these are found to be within the range. In case of PPC, the energy used for these major contributors is found to be lower compared to several other regions reported in Table 1. It is observed that the energy use for OPC is higher than the energy use for clinker because of the additional energy required for grinding, packing and other processes. The energy use for PPC is less than the energy use for OPC due to reduced clinker content. For case study 1, the energy use for PPC is 23.4% less compared to OPC due to reduced clinker content. For case study 2, the energy use for PPC is 28.5% less compared to OPC.

Table 5 Energy associated with OPC and PPC (MJ/ton of cement)

Table 6 presents CO₂ emissions generated from OPC and PPC for both case studies. The major sources of emission are the clinker and electricity used for grinding and packing. These findings are compared with the emission data from other regions presented in Table 2. It is observed that CO₂ emissions generated from OPC for case study 1 is lower than all regions except region 4. In case of case study 2, CO₂ emissions generated from OPC are less than regions 1 and 5 and higher than the remaining regions. In case of PPC, CO₂ emissions due to clinker and electricity use are found to be lower than all regions except region 4 in both case studies. CO₂ emissions per unit quantity of OPC are less than clinker due to reduced clinker quantity and high CO₂ intensity of clinker. For case study 1, CO₂ emissions from PPC are 24.4% less compared to OPC due to reduced clinker content. For case study 2, CO₂ emissions from PPC are 30.4% less compared to OPC.

Table 6 CO₂ associated with OPC and PPC

The energy use for OPC is nearly equal to the energy use for clinker due to the fact that about 95% of OPC is represented by clinker. Further, the energy use for processes like grinding and packing is added in estimating the energy use for OPC. This makes the total energy use for OPC more than or equal to the clinker. Similar calculation is applicable for determining CO₂ emissions. Since CO₂ intensity is higher for clinker, the net CO₂ emissions for OPC is less than clinker even after considering CO₂ associated with grinding and packing.

Conclusions

This study focused on collecting a set of inventory data (inputs and outputs) corresponding to the manufacturing of clinker and cement (Tables 7, 8, 9, 10, 11 and 12 of “Appendix”). Determination of energy use and CO₂ emissions is one of the outcomes of this study. Inventory data are the base for determining the energy use, CO₂ emissions, and environmental impacts using the sustainability assessment tools and methods. Inventory data can be used for manifold purposes, for example, calculation of water use, identification of toxic air emissions and the type and the quantity of waste generated. Studies on inventory data for clinker and cement manufacturing are well established in Europe and North America. In India, the inventory data, total energy use and total CO₂ emissions related to manufacturing of clinker, OPC and PPC are not directly available with a gate-to-gate system boundary condition considering all processes from mining to packing. This study attempts to address this gap.

Table 7 Energy use for Clinker (case study 1)

Table 8 CO₂ emissions for Clinker (case study 1)

Table 9 Energy use for OPC (case study 1)

Table 10 CO₂ emissions for OPC (case study 1)

Table 11 Energy use for PPC (case study 1)

Table 12 CO₂ emissions for PPC (case study 1)

The Cement Sustainability Initiative database represents the production data with a coverage of 49% of the total cement production in India. However, there is a need to transform this in the form of inventory data as per ISO 14044:2006 [13] tracking all processes from mining to packing within the gate-to-gate boundary condition. Most of the collected data from cement plants are in the form of absolute consumption per year, average rate of material flow per intermediate product and in miscellaneous forms.

The novelty of this work is: (1) calculation of energy use and CO₂ emissions for clinker, OPC and PPC instead of a generic cement based on two Indian case studies. The energy use and CO₂ emissions are selected as these two indicators are widely used in practice; (2) breakup of energy use and emissions is presented by data type (for example, raw materials, energy—thermal and electrical, outputs and emissions to air) and process-wise (for example, limestone extraction, limestone processing and crushing, other raw materials processing, raw meal preparation, fuel preparation, clinkerization, grinding, packaging and other miscellaneous processes) which facilitates a clear understanding of the major contributors; (3) clear definition of system boundary condition used for calculation, i.e. gate-to-gate boundary considering all processes from mining to packing; and (4) this study presents the life-cycle inventory data used for calculation of energy use and CO₂ emissions as per the ISO 14044:2006 [13] standard.

The energy use and CO₂ emissions are presented in the order of case study 1 and case study 2 as follows: (1) energy use (clinker): 3990 and 3626 MJ/ton; (2) CO₂ emissions (clinker): 849 and 868 kg CO₂/ton; (3) energy use (OPC): 4015 and 3821 MJ/ton; (4) energy use (PPC): 3077 and 2733 MJ/ton; (5) CO₂ emissions (OPC): 802 and 855 kg CO₂/ton; and (6) CO₂ emissions (PPC): 606 and 595 kg CO₂/ton. It is noted that the energy use and CO₂ emissions of PPC are at least 20% less compared to OPC. The thermal energy used for clinkerization and the electricity used for clinkerization, raw meal preparation, fuel preparation and limestone crushing together are found to be the major sources of energy use in clinker production. Direct CO₂ emissions generated from raw meal are the major source of emissions in clinker production. This is followed by other sources such as the thermal energy used for clinkerization. Clinker and the electricity used for grinding and packing are identified as major contributors of energy use and emissions in the manufacturing of OPC and PPC.

Appendix

Calculation of energy use and CO₂ emissions for clinker, OPC and PPC using the classified inventory data.

See Tables 7, 8, 9, 10, 11and 12.