Conventional and Alternative Sources of Thermal Energy in the Production of Cement—An Impact on CO2 Emission

The article evaluates the reduction of carbon dioxide emission due to the partial substitution of coal with alternative fuels in clinker manufacture. For this purpose, the calculations were performed for seventy waste-derived samples of alternative fuels with variable calorific value and variable share in the fuel mixture. Based on annual clinker production data of the Polish Cement Association and the laboratory analysis of fuels, it was estimated that the direct net CO2 emissions from fossil fuel combustion alone were 543 Mg of CO2 per hour. By contrast with the full substitution of coal with alternative fuels (including 30% of biomass), the emission ranged from 302 up to 438 Mg of CO2 per hour, depending on fuel properties. A reduction of 70% in the share of fossil fuels resulted in about a 23% decrease in net emissions. It was proved that the increased use of alternative fuels as an additive to the fuel mix is also of economic importance. It was determined that thanks to the combustion of 70% of alternative fuels of calorific value from 15 to 26 MJ/kg, the hourly financial profit gain due to avoided CO2 emission and saved 136 megatons of coal totaled an average of 9718 euros. The results confirmed that the co-incineration of waste in cement kilns can be an effective, long-term way to mitigate carbon emissions and to lower clinker production costs. This paper may constitute a starting point for future research activities and specific case studies in terms of reducing CO2 emissions.


Introduction
Cement manufacturing constitutes a significant source of anthropogenic CO 2 emissions both in the European Union and in the world [1]. The sector is responsible for approximately 5-9 percent of worldwide emissions [2,3]. In 2010-2019, carbon dioxide coming from the Polish cement industry constituted about 2-3% of the country's total CO 2 emissions [4,5]. Between 2010 and 2016, the CO 2 emission in the cement sector remained at an average level of 9.5 million tons, except for the increase to the level of 11.43 million tons in 2011, related to the record results in the construction industry in terms of the number of implemented projects. After 2016, a slow increase in the concentration of carbon dioxide from clinker production was observed, again to the level of 11.29 million tons in 2019, which was conditioned by economic growth. Based on the annual production of clinker and cement for this period [5], it can be estimated that circa 700-800 kg of CO 2 was emitted per ton of clinker produced and about 500-700 kg of CO 2 per one ton of cement produced.
In a cement plant, the calcination process plays an important role as concerns the environmental impact. It results from the fact that around 50-60% [6] of the CO 2 is liberated directly during the reaction of thermal decomposition of limestone. The combustion of fossil fuels in cement kilns contributes to about 40% of the carbon dioxide emissions [7], goals should focus on the further valorization of waste in the production process (i.e., the use of waste as an alternative fuel and a raw material in clinker production).
The scope of this work was to determine the direct CO 2 emission from co-combustion of the fossil fuel and alternative fuels. From the point of view of environmental protection, this constitutes an urgent issue to be addressed. The calculations were performed based on the amount of fuels needed for annual clinker production data and the CO 2 emission factor of the fuels. In the analyzed case, various configurations of fuel co-combustion were assumed; secondly, hard coal and alternative fuels were burned only in the main burner of the rotary kiln. Seventy samples of alternative fuels were analyzed to investigate how the quality parameters of various alternative fuels may affect the final amount of CO 2 emission. The modified parameters of the samples included the calorific value and the carbon content. In the literature, the correlation between AF parameters and the amount of CO 2 emissions has been poorly discussed. Previously, the researchers [26][27][28] have focused primarily on studying the changes of clinker reactivity resulting from the use of various alternative fuels in the production of cement. In this work, an economic effect achieved due to avoided CO 2 emission and the mass saving of the fossil fuel was additionally calculated.

Cement Manufacturing Process
The worldwide cement production in 2019 reached the level of 4 billion tons, this was an increase of approximately 50% as compared with 2005 production. Until now, China has been the largest cement producer by installed capacity manufacturing over half of the world's cement, with India as the second global producer [29]. World cement production in 2018, by regions and major countries, is presented in Figure 1. Since 2017, Poland has been the third-largest producer of cement in Europe, after Germany and Italy. Cement production in Poland in 2019 amounted to 19.0 million tons, which is 10% more than in 2017. EU goals should focus on the further valorization of waste in the production process (i.e., the use of waste as an alternative fuel and a raw material in clinker production). The scope of this work was to determine the direct CO2 emission from co-combustion of the fossil fuel and alternative fuels. From the point of view of environmental protection, this constitutes an urgent issue to be addressed. The calculations were performed based on the amount of fuels needed for annual clinker production data and the CO2 emission factor of the fuels. In the analyzed case, various configurations of fuel co-combustion were assumed; secondly, hard coal and alternative fuels were burned only in the main burner of the rotary kiln. Seventy samples of alternative fuels were analyzed to investigate how the quality parameters of various alternative fuels may affect the final amount of CO2 emission. The modified parameters of the samples included the calorific value and the carbon content. In the literature, the correlation between AF parameters and the amount of CO2 emissions has been poorly discussed. Previously, the researchers [26][27][28] have focused primarily on studying the changes of clinker reactivity resulting from the use of various alternative fuels in the production of cement. In this work, an economic effect achieved due to avoided CO2 emission and the mass saving of the fossil fuel was additionally calculated.

Cement Manufacturing Process
The worldwide cement production in 2019 reached the level of 4 billion tons, this was an increase of approximately 50% as compared with 2005 production. Until now, China has been the largest cement producer by installed capacity manufacturing over half of the world's cement, with India as the second global producer [29]. World cement production in 2018, by regions and major countries, is presented in Figure 1. Since 2017, Poland has been the third-largest producer of cement in Europe, after Germany and Italy. Cement production in Poland in 2019 amounted to 19.0 million tons, which is 10% more than in 2017. Cement is a hydraulic binder, which means that after mixing with water it sets, hardens, and achieves proper strength characteristics, even in underwater conditions [30,31]. Due to its properties, cement is widely used as a binding material in the construction industry performing the role of a component of concrete mixtures, mortars, plasters, and many other products of construction chemistry. This finely ground material of gray color is produced by grinding clinker with calcium sulfate, being a setting time regulator (in the form of gypsum or anhydrite), and various ingredients such as granulated blast furnace slag, fly ash, or limestone (depending on the type of cement) in a cement mill [32]  Cement is a hydraulic binder, which means that after mixing with water it sets, hardens, and achieves proper strength characteristics, even in underwater conditions [30,31]. Due to its properties, cement is widely used as a binding material in the construction industry performing the role of a component of concrete mixtures, mortars, plasters, and many other products of construction chemistry. This finely ground material of gray color is produced by grinding clinker with calcium sulfate, being a setting time regulator (in the form of gypsum or anhydrite), and various ingredients such as granulated blast furnace slag, fly ash, or limestone (depending on the type of cement) in a cement mill [32]. Portland clinker is obtained by burning ground raw materials in a rotary kiln. The five stages which can be distinguished in the clinker production process occur in the following order: (1) the dehydration process (heating and drying of the homogenized raw materials), (2) the calcination process (decomposition of raw materials into calcium oxide and carbon dioxide [33,34], (3) the clinkerization process (formation of clinker phases: tricalcium silicate (C 3 S), dicalcium silicate (C 2 S), tricalcium aluminate (C 3 A) and tetracalcium aluminoferrite (C 4 AF) and (4) the clinker cooling process. The cement manufacturing process, along with marked sources of pollutant emissions, is schematically displayed in Figure 2 [32]. is obtained by burning ground raw materials in a rotary kiln. The five stages which can be distinguished in the clinker production process occur in the following order: (1) the dehydration process (heating and drying of the homogenized raw materials), (2) the calcination process (decomposition of raw materials into calcium oxide and carbon dioxide [33,34], (3) the clinkerization process (formation of clinker phases: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) and (4) the clinker cooling process. The cement manufacturing process, along with marked sources of pollutant emissions, is schematically displayed in Figure 2 [32].

Materials and Methods
In the present work, 70 alternative fuel (AF) samples, understood as "solid secondary fuel" according to the EN 15357: 2011 standard, were analyzed to determine the potential reduction of direct CO2 emission from the co-firing of waste-derived alternative fuels. Samples were derived by Cement Plant, Poland. The types of fuels covered in the study are mixtures of non-hazardous high-calorie waste such as plastics, paper, textiles, and tires, coming from the mechanical treatment of waste (for example sorting, crushing, compacting, pelletizing) [17]. An analysis of AF quality parameters was performed in the Department of Environmental Monitoring, Central Mining Institute. The samples tested were initially dried at the temperature of 313 K. The dried samples were ground with the application of a knife mill (LMN-100, TESTCHEM); the nominal grain diameter in the prepared sample did not exceed 2 mm. The test samples were obtained by grinding with the use of a cryogenic mill (6870, SPEX SamplePrep LLC, Metuchen, NJ, USA). The total carbon content in the samples was determined by means of the high-temperature combustion method with IR detection (HELIOS CHS 900, ELTRA), in accordance with standard EN 15407:2011. The calorific value and the heat of combustion were determined using PM Inputs: Emission: E-electrical energy PM-particular matter H-thermal energy GAS-gases emission

Materials and Methods
In the present work, 70 alternative fuel (AF) samples, understood as "solid secondary fuel" according to the EN 15357:2011 standard, were analyzed to determine the potential reduction of direct CO 2 emission from the co-firing of waste-derived alternative fuels. Samples were derived by Cement Plant, Poland. The types of fuels covered in the study are mixtures of non-hazardous high-calorie waste such as plastics, paper, textiles, and tires, coming from the mechanical treatment of waste (for example sorting, crushing, compacting, pelletizing) [17]. An analysis of AF quality parameters was performed in the Department of Environmental Monitoring, Central Mining Institute. The samples tested were initially dried at the temperature of 313 K. The dried samples were ground with the application of a knife mill (LMN-100, TESTCHEM); the nominal grain diameter in the prepared sample did not exceed 2 mm. The test samples were obtained by grinding with the use of a cryogenic mill (6870, SPEX SamplePrep LLC, Metuchen, NJ, USA). The total carbon content in the samples was determined by means of the high-temperature combustion method with IR detection (HELIOS CHS 900, ELTRA), in accordance with standard EN 15407:2011. The calorific value and the heat of combustion were determined using the calorimetric method (C 5000 IKA WERKE), based on the PN-EN 15400:2011 standard. The results of the tests of AF samples are presented in Figure 3.  The total emission of CO2 coming from the co-combustion of coal and alternative fuels in a cement kiln was calculated using the following equation [35]: where: , is CO2 emission from the combustion process by type of fuel in Mg CO2 per hour; is the amount of fuel combusted mass converted into the energy content of this fuel in TJ per hour; is the CO2 emission factor in Mg CO2/TJ.
The carbon dioxide emission factor from fuel combustion expressed in megatons of CO2 emitted per unit of calorific value is calculated according to equation [35]: where: is the CO2 emission factor in Mg CO2/TJ; 3.664 is a molecular weight ratio of CO2 to C; is carbon content of combusted fuel in %; is calorific value in TJ/Mg. The total emission from the combustion process was a sum of CO2 emissions from all fuels used, i.e., from coal and alternative fuels.
The calculation was performed for several scenarios with various shares of alternative fuels. The fuel mix composition data are presented in Table 1. For the purpose of this work, the following assumptions were made as listed below: The total emission of CO 2 coming from the co-combustion of coal and alternative fuels in a cement kiln was calculated using the following equation [35]: where: E CO 2, f uel is CO 2 emission from the combustion process by type of fuel in Mg CO 2 per hour; Z is the amount of fuel combusted mass converted into the energy content of this fuel in TJ per hour; EF CO 2 is the CO 2 emission factor in Mg CO 2 /TJ.
The carbon dioxide emission factor from fuel combustion expressed in megatons of CO 2 emitted per unit of calorific value is calculated according to equation [35]: where: EF CO 2 is the CO 2 emission factor in Mg CO 2 /TJ; 3.664 is a molecular weight ratio of CO 2 to C; C is carbon content of combusted fuel in %; Q is calorific value in TJ/Mg.
The total emission from the combustion process was a sum of CO 2 emissions from all fuels used, i.e., from coal and alternative fuels.
The calculation was performed for several scenarios with various shares of alternative fuels. The fuel mix composition data are presented in Table 1. For the purpose of this work, the following assumptions were made as listed below: (a) The yearly average heat demand for the production of clinker equals 45 × 10 9 MJ. The value was calculated from the annual clinker production data for the period of 2010-2018, published in the Polish Cement Association report [4], and using the following Equation (3): where: H d is the average CO 2 total heat demand to produce clinker in MJ per year; M c is average capacity of cement kiln in Mg per year; H s is specific heat consumption per ton of clinker in MJ/Mg.
The results of the Polish Cement Association report are summarized in Figure 4. As seen, the specific heat consumption per ton of clinker ranged from 3677 to 3828 MJ/Mg. Thus, it was calculated that with an average capacity of cement kiln of about 12 × 10 6 Mg of clinker, it is necessary to provide approximately 44-46 × 10 9 MJ of heat. (a) The yearly average heat demand for the production of clinker equals 45 × 10 9 MJ. The value was calculated from the annual clinker production data for the period of 2010-2018, published in the Polish Cement Association report [4], and using the following Equation (3): where: is the average CO2 total heat demand to produce clinker in MJ per year; is average capacity of cement kiln in Mg per year; is specific heat consumption per ton of clinker in MJ/Mg. The results of the Polish Cement Association report are summarized in Figure 4. As seen, the specific heat consumption per ton of clinker ranged from 3677 to 3828 MJ/Mg. Thus, it was calculated that with an average capacity of cement kiln of about 12 × 10 6 Mg of clinker, it is necessary to provide approximately 44-46 × 10 9 MJ of heat. (f) In the performed calculation, it was assumed that the biogenic fraction in alternative fuel samples will be 30% [37,38].
(g) The price of CO2 emission allowances is 30.

Alternative Fuels Characteristic
The results of the qualitative analysis (see Figure 3) indicate differences between individual samples of alternative fuel, which may result from the heterogeneous composition of the waste stream used for fuel production. Alternative fuels are mainly produced from mixed streams of municipal solid waste, including different shares of plastic, paper, textile, or rubber. A typical composition of AF regarding its different components is presented in works [20,[40][41][42]. The selected groups of wastes are characterized by various physical-chemical parameters [43,44]. Plastics have a very high calorific value, even exceeding 40 MJ/kg, due to a low content of ash and moisture, while paper and wood has the calorific value of about 11-20 MJ/kg. For comparison, the heating value of coal is about 28 MJ/kg. The share of a given fraction in the alternative fuel determines its calorific value.  (f) In the performed calculation, it was assumed that the biogenic fraction in alternative fuel samples will be 30% [37,38].

Alternative Fuels Characteristic
The results of the qualitative analysis (see Figure 3) indicate differences between individual samples of alternative fuel, which may result from the heterogeneous composition of the waste stream used for fuel production. Alternative fuels are mainly produced from mixed streams of municipal solid waste, including different shares of plastic, paper, textile, or rubber. A typical composition of AF regarding its different components is presented in works [20,[40][41][42]. The selected groups of wastes are characterized by various physicalchemical parameters [43,44]. Plastics have a very high calorific value, even exceeding 40 MJ/kg, due to a low content of ash and moisture, while paper and wood has the calorific value of about 11-20 MJ/kg. For comparison, the heating value of coal is about 28 MJ/kg. The share of a given fraction in the alternative fuel determines its calorific value.  54.83%, and 60.51%, respectively. As shown in Figure 3, the higher carbon content in individual AF samples does not always correspond with a higher calorific value. For example, the carbon content in sample 18 (with the calorific value of 18.12 MJ/kg) is about 55.24%, and in samples 67 (with the calorific value of 24.87 MJ/kg) and 68 (with the calorific value of 25.07 MJ/kg) the carbon content is 52.38% and 59.11%, respectively.

Mass Balance in Co-Combustion of Coal and Alternative Fuel
The analyzed case demonstrated that for the production of 12 thousand Mg of clinker per year, with a unit heat consumption of 3732 MJ/Mg, an amount of 194.31 Mg of coal per hour is needed. Figure 5 shows the quantity of alternative fuel necessary to obtain the required amount of heat, i.e., about 5.6 TJ per hour, with varying levels of coal substitution.  54.83%, and 60.51%, respectively. As shown in Figure 3, the higher carbon content in individual AF samples does not always correspond with a higher calorific value. For example, the carbon content in sample 18 (with the calorific value of 18.12 MJ/kg) is about 55.24%, and in samples 67 (with the calorific value of 24.87 MJ/kg) and 68 (with the calorific value of 25.07 MJ/kg) the carbon content is 52.38% and 59.11%, respectively.

Mass Balance in Co-Combustion of Coal and Alternative Fuel
The analyzed case demonstrated that for the production of 12 thousand Mg of clinker per year, with a unit heat consumption of 3732 MJ/Mg, an amount of 194.31 Mg of coal per hour is needed. Figure 5 shows the quantity of alternative fuel necessary to obtain the required amount of heat, i.e., about 5.6 TJ per hour, with varying levels of coal substitution. Figure 5. The mass of the alternative fuel is required for the production of clinker. The substitution of coal with alternative fuels was: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%.
As can be seen in Figure 5, an hourly demand for alternative fuels decreases with the increase in calorific value. Assuming complete substitution of coal with the fuel of the AF type, the necessary amount of an alternative fuel with a calorific value of 15-19 MJ/kg is 300-400 Mg/h, while for an alternative fuel with a calorific value of 19-26 MJ/kg-it is 200-300 Mg/h. The effect of calorific value on the amount of the alternative fuel consumed is less noticeable at the consumption of 70-80% coal in the clinker production. For example, at 20% co-combustion of AF, the difference between the amount of the low calorie and high-calorie fuels is 31 Mg/h, while at 80% substitution by AF the difference is 123 Mg/h. It was calculated that the thermal deficit due to the reduction of coal burning to 40% makes that 2-3 times more alternative fuel must be mass transported into the rotary kiln than in the case of the conventional fuel, while with a 20% reduction it is 5-8 times more. This indicates that the alternative fuel storage area must be located significantly higher or the frequency of fuel supply increased as compared to coal.
In this study, the mass savings of coal achieved due to a partial replacement with alternative fuel are presented in Table 2. Based on the results, it can be estimated that the substitution of coal with alternative fuels at the level of 70% means that the savings for the cement producer may exceed 55 million euros per year. It should be noted that to calculate the total financial profit, it is necessary to deduct the costs of alternative fuel from the value given in Table 2. The price of alternative fuels is much lower compared to the costs associated with coal, which makes these fuels more cost-effective [45,46].   Figure 5. The mass of the alternative fuel is required for the production of clinker. The substitution of coal with alternative fuels was: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%.
As can be seen in Figure 5, an hourly demand for alternative fuels decreases with the increase in calorific value. Assuming complete substitution of coal with the fuel of the AF type, the necessary amount of an alternative fuel with a calorific value of 15-19 MJ/kg is 300-400 Mg/h, while for an alternative fuel with a calorific value of 19-26 MJ/kg-it is 200-300 Mg/h. The effect of calorific value on the amount of the alternative fuel consumed is less noticeable at the consumption of 70-80% coal in the clinker production. For example, at 20% co-combustion of AF, the difference between the amount of the low calorie and high-calorie fuels is 31 Mg/h, while at 80% substitution by AF the difference is 123 Mg/h. It was calculated that the thermal deficit due to the reduction of coal burning to 40% makes that 2-3 times more alternative fuel must be mass transported into the rotary kiln than in the case of the conventional fuel, while with a 20% reduction it is 5-8 times more. This indicates that the alternative fuel storage area must be located significantly higher or the frequency of fuel supply increased as compared to coal.
In this study, the mass savings of coal achieved due to a partial replacement with alternative fuel are presented in Table 2. Based on the results, it can be estimated that the substitution of coal with alternative fuels at the level of 70% means that the savings for the cement producer may exceed 55 million euros per year. It should be noted that to calculate the total financial profit, it is necessary to deduct the costs of alternative fuel from the value given in Table 2. The price of alternative fuels is much lower compared to the costs associated with coal, which makes these fuels more cost-effective [45,46].

CO 2 Emission Balance
According to Equation (2) and qualitative data of coal sample (see Section 3) the CO 2 emission factor of coal was calculated as The emission factor for all AF samples tested was calculated in a similar way, the results are summarized in Figure 6. The determined factor values of alternative fuel ranged from 77.22 Mg CO 2 /TJ to 111.78 Mg CO 2 /TJ (the average value being 91.91 Mg CO 2 /TJ). Uncertainty estimates for CO 2 emission factors for the fossil fuel and alternative fuels were 1.5% and 3-4%, respectively. As can be seen, the factor value of some AF samples exceeds the emission factor from the combustion of coal. Higher emission factor values were recorded for sixteen fuel samples (numbered 8, 11, 13, 18, 21, 22, 25, 28, 29, 30, 32, 33, 35, 36, 42, and 50), mainly with a low calorific value of below 20 MJ/kg and a high carbon content (>52%). The ratio of carbon content and calorific value varied from 26.93 to 30.49 for these selected samples of AF, while for coal it was 26.47. According to [10], the emission factor for hard coal (for the year 2019), calculated based on the national average calorific value, is 94.70, i.e., the ratio of carbon to the calorific value is 25.85. This means that the most effective way to achieve a low CO 2 emission factor will be to use alternative fuels with a ratio value below 25-26.

CO2 Emission Balance
According to Equation (2) and qualitative data of coal sample (see Section 3) the CO2 emission factor of coal was calculated as The emission factor for all AF samples tested was calculated in a similar way, the results are summarized in Figure 6. The determined factor values of alternative fuel ranged from 77.22 MgCO2/TJ to 111.78 MgCO2/TJ (the average value being 91.91 MgCO2/TJ). Uncertainty estimates for CO2 emission factors for the fossil fuel and alternative fuels were 1.5% and 3-4%, respectively. As can be seen, the factor value of some AF samples exceeds the emission factor from the combustion of coal. Higher emission factor values were recorded for sixteen fuel samples (numbered 8, 11, 13, 18, 21, 22, 25, 28, 29, 30, 32, 33, 35, 36, 42, and 50), mainly with a low calorific value of below 20 MJ/kg and a high carbon content (>52%). The ratio of carbon content and calorific value varied from 26.93 to 30.49 for these selected samples of AF, while for coal it was 26.47. According to [10], the emission factor for hard coal (for the year 2019), calculated based on the national average calorific value, is 94.70, i.e., the ratio of carbon to the calorific value is 25.85. This means that the most effective way to achieve a low CO2 emission factor will be to use alternative fuels with a ratio value below 25-26.    Biogenic carbon emission comes from the waste of biological origins such as residues or waste streams from forestry and timber processing, agriculture, pulp, and paper as well as sugar industries [45,48]. The biogenic fraction in a given fuel is determined by appropriate laboratory tests specified in European standards, i.e., the manual sorting method (MS), the selective dissolution method (SDM), or the radiocarbon method (14C-Method) [21,40].
In accordance with the Intergovernmental Panel on Climate Change guidelines, the emission of biogenic carbon in the form of carbon dioxide from the incineration process is regarded as climate-neutral since carbon is generated by the natural cycle, and can be excluded from the total amount of CO2 emissions [49,50]. The majority of case studies assumed that the amount of CO2 absorbed by growing forests through the photo-synthesis process is equal to the emission from the combustion process [51,52]. However, in recent years, there has been a more and more common belief that biomass fuels should not be considered carbon neutral when forest energy resource management is not carried out sustainably [53][54][55]. In this work, it was assumed that carbon dioxide emissions from biomass fraction can be considered carbon neutral.
As can be seen in Figure 7, a co-firing of coal with 30% of alternative fuels allows one to avoid about 40-59 Mg of CO2 emission per hour, while with 80% and 100% of AF even 104-150 Mg of CO2 or 130-188 Mg of CO2 per hour, respectively. The curves show slight decreases in the amount of avoided emission with the increases in calorific value of the alternative fuel samples (the samples are numbered from the lowest to the highest calorific value), particularly with high coal to AF substitution. It can be related to the fact that a necessary amount of a high-calorific AF to supplement the thermal deficit is lower than that of a low-calorific fuel. The irregular trend of the curves and the occurrence of peaks are related to the variability of quality parameters in the analyzed fuel samples, especially the carbon content. For example, the carbon content of fuels with a calorific value of about 18 MJ/kg varies from 39.17% to 55.36%. Table 3 shows the financial benefits resulting from the avoided fees for CO2 emissions. In the analyzed case, the cost of CO2 avoided ranges from 465 euros per hour for the lowest reduction of coal combusted to 4185 euros per hour (assuming 90% combustion of the alternative fuel). On an annual basis, these costs will amount to several million euros. Of course, the greater the biogenic fraction share is, the greater the profit. With the share of the biogenic fraction at the level of 60%, the savings in Table 3    Biogenic carbon emission comes from the waste of biological origins such as residues or waste streams from forestry and timber processing, agriculture, pulp, and paper as well as sugar industries [45,48]. The biogenic fraction in a given fuel is determined by appropriate laboratory tests specified in European standards, i.e., the manual sorting method (MS), the selective dissolution method (SDM), or the radiocarbon method (14C-Method) [21,40].
In accordance with the Intergovernmental Panel on Climate Change guidelines, the emission of biogenic carbon in the form of carbon dioxide from the incineration process is regarded as climate-neutral since carbon is generated by the natural cycle, and can be excluded from the total amount of CO 2 emissions [49,50]. The majority of case studies assumed that the amount of CO 2 absorbed by growing forests through the photo-synthesis process is equal to the emission from the combustion process [51,52]. However, in recent years, there has been a more and more common belief that biomass fuels should not be considered carbon neutral when forest energy resource management is not carried out sustainably [53][54][55]. In this work, it was assumed that carbon dioxide emissions from biomass fraction can be considered carbon neutral.
As can be seen in Figure 7, a co-firing of coal with 30% of alternative fuels allows one to avoid about 40-59 Mg of CO 2 emission per hour, while with 80% and 100% of AF even 104-150 Mg of CO 2 or 130-188 Mg of CO 2 per hour, respectively. The curves show slight decreases in the amount of avoided emission with the increases in calorific value of the alternative fuel samples (the samples are numbered from the lowest to the highest calorific value), particularly with high coal to AF substitution. It can be related to the fact that a necessary amount of a high-calorific AF to supplement the thermal deficit is lower than that of a low-calorific fuel. The irregular trend of the curves and the occurrence of peaks are related to the variability of quality parameters in the analyzed fuel samples, especially the carbon content. For example, the carbon content of fuels with a calorific value of about 18 MJ/kg varies from 39.17% to 55.36%. Table 3 shows the financial benefits resulting from the avoided fees for CO 2 emissions. In the analyzed case, the cost of CO 2 avoided ranges from 465 euros per hour for the lowest reduction of coal combusted to 4185 euros per hour (assuming 90% combustion of the alternative fuel). On an annual basis, these costs will amount to several million euros. Of course, the greater the biogenic fraction share is, the greater the profit. With the share of the biogenic fraction at the level of 60%, the savings in Table 3 will double. The financial benefits will also depend on the price of carbon dioxide emission allowances of the Emission Trading System (EU ETS) [39]. From the beginning of 2020 to mid-March, the prices for the emission of 1 Mg of CO 2 were at the level of 22-26 euros. At the end of March 2020, the prices of CO 2 emission allowances dropped significantly, even to 14 euros. Currently, the exchange price of CO 2 emission allowances reaches the level of 28-30 euros.
In the analyzed case, each increase in the share of heat from alternative fuels by 10% means on average about 15 Mg per hour of additional CO 2 emissions avoided.  Figure 8 presents the total gross and net emissions of CO 2 . The gross emission is related to the Mg of CO 2 emitted from coal and the alternative fuel together, while net emission takes into account a zero-emission factor from the biodegradable fraction in AF. The calculations were made on the assumption of two different biogenic fraction contents in fuels, namely 30% and 60%. The higher content of the biogenic fraction was assumed based on the results of fuel characteristics from various cement plants, presented in work [20,21]. The mean gross emission factor, at the 10% alternative fuels share in the mix, was 540.21 Mg CO 2 per hour, and at the 90% alternative fuels share-it declined to the level of 518.71 Mg CO 2 per hour. Uncertainty estimates for the emissions measurement of gross carbon dioxide were on the order of 4-5%. For comparison, to obtain 5.6 TJ·h −1 of heat, 194.31 Mg of coal per hour is needed, which corresponds to the total emission from coal combustion of about 542.90 Mg CO 2 . As can be observed in Figure 8, the replacement of coal with AF has undoubtedly resulted in a reduction of fuel emissions, particularly when the emission from biogenic fraction was considered carbon neutral. It means that the use of biomass and biomass by-products as alternative fuels can be classified as limiting CO 2 emission. The mean net emission factor ranged from 524.73 Mg CO 2 per hour up to 379.38 Mg CO 2 per hour. It was found that each successive 10% increase in the share of heat from alternative fuels (with the biogenic fraction of 30%) contributed to the reduction of the net emission level by each subsequent 3-4%, in relation to the emission from the combustion of coal alone. In the case of 90% coal and 10% alternative fuels, the average level of net emissions from all samples is 524.73 Mg CO 2 , and with 80% coal and 20% alternative fuels it is 506.56 Mg CO 2 , whereas with 70% coal and 30% alternative fuels it is 488.39 Mg CO 2 . With regard to alternative fuels with a twice higher share of the biogenic fraction, each 10% replacement of coal will reduce emissions by about 6%, compared to coal emission. With the lowest share of alternative fuels (10%), the net emission varied from 505.90 Mg CO 2 per hour to 513.64 Mg CO 2 per hour, while with the higher share of alternative fuels (90%), the minimum net emission was 209.91 Mg CO 2 per hour and the maximum was 279.56 Mg CO 2 per hour. Based on the results, it was found that with the full substitution of coal, the average gross emission of CO 2 from all samples is 30% higher than the net emission (Figure 8j), while with 20% substitution of coal the difference is 6% (Figure 8b).  According to the methodology used for EU-ETS benchmarks, a limit value for clinker production is 766 kg CO2 per Mg clinker [56]. Based on the results shown in Figure 8, gross fuel emission is on average at the level of 353 kg CO2 per Mg of clinker (with the produc- According to the methodology used for EU-ETS benchmarks, a limit value for clinker production is 766 kg CO 2 per Mg clinker [56]. Based on the results shown in Figure 8, gross fuel emission is on average at the level of 353 kg CO 2 per Mg of clinker (with the production of 12 million Mg of clinker per year and the working time of 8000 h). Substitution of 50% and 70% coal with alternative fuels of 30% biomass content resulted in the reduction of fuel emissions to the level of 301 kg CO 2 and 277 kg CO 2 per Mg of clinker (net emission), respectively. This is about 30-40% of the emission benchmark of 766 kg CO 2 per Mg of clinker. It means that approximately 60-70% of CO 2 emissions can come from the calcination process (process emission).
A slight decline of the total net emissions is observed with the increase in the calorific value of the alternative fuels, particularly at the high level of coal substitution with AF (Figure 8h-j). For example, in the case of a 70% share of alternative fuels with the emission factors below 96 Mg CO 2 /TJ (Figure 6), the average net CO 2 emission from samples with a calorific value of 16-17 MJ/kg is 413 Mg CO 2 /hour, while for samples with a higher calorific value in the range of 21-22 MJ/kg and 24-25 MJ/kg, the emissions are 407 Mg CO 2 /h and 386 Mg CO 2 /h, respectively. For the sixteen remaining samples with the emission factor above 96 Mg CO 2 /TJ, the net emission was in the range of 430-470 Mg CO 2 /h and was only about 13-23% lower than the emission coming from 100% coal combustion, regardless of the calorific value.

Conclusions
The paper discusses the issue of using alternative fuels in clinker burning systems. The performed calculations showed the environmental and financial benefits of using alternative fuels as coal substitutes in cement manufacturing.
The estimated emissions were determined using the CO 2 emission factor for coal of 96.43 Mg CO 2 . The calculation reveals that the net emission of CO 2 was reduced by about 25%, which was achieved by the co-combustion of 70% alternative fuels with a CO 2 emission factor below 96.43 Mg CO 2 . In the case of fuel samples with a CO 2 emission factor above 96.43 Mg CO 2 , the decline in emissions was slight, about 18%. In the analyzed case, an increase in the share of alternative fuels by each 10% resulted in a saving of coal mass by 19.43 Mg per hour, which generates savings of about 923 euros. In turn, the 10% reduction of coal burned means that about 0.56 TJ/h of thermal deficit must be supplemented with alternative fuel. This requires the burning of about 28-37 Mg of AF with the calorific value <20 MJ/kg and about 22-27 Mg of AF with the calorific value <20 MJ/kg within an hour. The total savings of approximately 1388 euros per hour were calculated for each 10% increase in alternative fuels. The cost reduction resulted from both the mass savings of the coal and the co-combustion of alternative fuels with the 30% biogenic fraction content.
It has been proven that substituting coal with alternative fuels is one of the most prospective solutions, in terms of environmental protection and mitigating climate change. Co-combustion of AF reduces the consumption of high-emission fuels, thus enabling the fulfillment of the obligations stipulated by the Paris Agreement of 2015, i.e., the achievement of the carbon neutrality targets to combat global climate change. At the same time, alternative fuels produced based on waste contribute to the implementation of the idea of a circular economy through using materials otherwise destined for landfills.
Balancing off the cement plant's environmental impact constitutes an urgent issue, particularly with the progressive maximization of energy consumption from waste-derived alternative fuels, and undoubtedly requires further detailed research. Future studies should focus on assessing the impact of the morphological composition of waste characterized by high heterogeneity on the amount of CO 2 emissions coming from the combustion process.