Environmental Impacts Evaluations of Different Alternative Fuel Substitution Rate Scenarios in Clinker Production

. Cement production is an energy-intensive industry that primarily relies on fossil fuels like coal and natural gas to meet energy needs. Extreme usage of fossil fuels leads to depletion of their source and higher greenhouse gas (GHG) emissions such as NOx, SOx, and CO 2 . Clinker, as the primary material for cement, is a product of the clinkerization process in the kiln system, where the utilization of fossil fuel happens massively. Pre-calciner, as a part of the kiln system, combusts around 60% of the fuel requirement in the kiln system. The calcination reaction occurs within the pre-calciner at 700 - 900 ˚C and produces over 50% of the emissions. Alternative fuels proved the capability to meet the energy demand and mitigate GHG emissions. Previous studies show Aspen Plus is one of the powerful software, able to simulate the calcination and combustion process realistically. The process model in this study uses data from one of the leading cement plants in Yemen. The main aim of this study is to evaluate the environmental impacts of alternative solid fuel mixture (Tires-derived "TDF" and Plastic waste "PW") and coal with various scenarios of substitution rate. It mainly concentrates on the environment, quality, and energy outputs. Based on the simulation results of the investigated model, in the implementation of 100% alternative fuels mixture scenarios, PW increased the moisture percentage, affecting the outlet temperature, While TDF has higher emissions than PW. Likewise, the 50% alternative fuels mixture with various substitution rates of coal has shown satisfactory results with a low amount of coal regarding the emissions percentages.


Introduction
Worldwide, the cement industry is a significant player in thermal energy consumption and emitting greenhouse gas (GHG) scenarios [1].It is one of the most energy-intensive when it comes to heat consumption and performance in the kiln system to produce clinker [2].The unit of clinker production is the most energy-intensive process, which estimates for more than 80% of the energy used in the whole process of cement production.It is also the most important in terms of emissions potential, product quality, and cost.Clinker can be produced by burning a mixture of materials, mainly Limestone, Silicon oxides, Aluminum, and Iron oxides.[3].
High temperatures in this process are required to transform that mixture of materials into cement clinker.The clinker production process is the most crucial because of some terms such as product quality, emissions potential, thermal energy efficiency, and cost [4].Preheater pre-calciner tower is taking a place at the clinker production to heat raw meal at a temperature where calcination or dissociation of CO2 begins, outside the kiln [1].The precalcination chamber is known as a pre-calciner wont to preheat and partially calcine the meal.during this stage, the preheated raw material is burnt to undergo a calcination reaction.the essential chemical process of cement manufacturing begins with the decomposition of CaCO3 at about 900 °C to calcium oxide (CaO, lime) and gaseous CO2 [4].
In-line calciner (ILC) is a type of pre-calciner that is fed with gases from both kiln exhaust and tertiary & pre-heater [1].The calcination reaction could be highly endothermic, with a heat requirement of 1784 Kj/kg of CaCO3 [5].This energy is supplied from the combustion of solid fuels together with the exhaust gases from a rotary kiln.Employing a pre-calciner within the system decreases the NOx emission and, consequently, the energy consumption of clinkerization by 8 to 11% [6].All calciners require fuel firing arrangement as well as raw meal feeding.The grate cooler sends hot air through a duct.The air used in the calciner is referred to as 'tertiary air,' and the duct that transports it is referred to as a tertiary air duct.To eliminate clinker dust, a dust chamber is placed between the throat area / t.a.duct and the grate cooler [7].
In 2018, the level of the global cement industry key indicator for thermal energy intensity of clinker was (3,4 GJ/t clinkers) [8].It is a bit far from the reported value of the best available technology performance levels (2,9-3,0 GJ/t clinkers for dry-process kilns with a pre-calciner and a six-stage cyclone pre-heater) [9].Manufacturing a ton of cement releases 0,73-0,99 tons of CO2, which depends on many factors such as the clinker to cement ratio.Dissimilar to other industries, energy consumption is not the predominant driver of CO2 emissions.In turn, over 50% of the emissions are the effect of the calcination process, wherein lime and carbon dioxide are the yields of cracking calcium carbonate to produce clinker [10].
A study in Indonesia regarding the relationship between energy consumption and CO2 emission shows how the increased consumption of non-green energy or fossil will increase the amount of carbon dioxide emissions; on the contrary, the consumption of green energy can reduce the amount of carbon dioxide emissions [11].Fossil fuels are no longer sufficient to meet energy demand [12] and are an environmentally unfriendly option [13].
In the last few years, there has been a growing interest in alternative fuels as a part of renewable energy and green production, which is one of the solutions to decrease (GHG), optimize the production process, and slightly minimize the cost of production.Although a wide range of technical constraints prevents alternative fuels from being used in cement plants, the variation of wastes that could potentially be used in the cement industry is vast.Aside from any processing limitations, the cement industry has developed international guidelines that list wastes that cannot be used as an alternative fuel, including radioactive waste, infectious waste, and explosives [14].Scientists and researchers have been working on alternative fuels to enhance clinker production by various methods without harming the quality, environment, energy efficiency improvement, and cost-saving opportunities to produce clinker [2].Proper alternative fuels are those with a high calorific value, chemical composition, which a well-known and consistent, and predictable availability.According to both research and international experience, no single alternative fuel can meet the entire thermal demand of cement manufacturing by itself.A combination of alternative fuels, on the other hand, can achieve that goal [14].
The application of such technology will require a cost of installation or simply conducting a simulation to demonstrate the feasibility of the alternative fuels [15].The chosen alternative fuels will then be put to the test via a simulation software such as Aspen plus [16].Several studies have examined various types and amounts of alternative fuels mixed with coal, focusing on several impacts such as energy [17], emissions [18][19], and precise burning zone specification [20].Ismail and Zieri proposed some scenarios in the mixture and substitution rate utilization in the ILC study about the alternative fuels from waste products in the cement industry [21].A preliminary simulation study is needed to investigate the Unity Cement Company (UCC) plant in Yemen, represented by the ILC model.The model will only consider the simulation of the Pre-calciner, the combustion process, and their reactions based on the provided data of the plant.It will represent the production process of clinker in this study to simplify its complexity.

Methodology
The main target of this study is to investigate the behaviors and impacts of the solid alternative fuel mixtures in the ILC with 60% of the total fuel consumption.It mainly concentrates on the environmental impacts evaluations of different alternative fuel substitution rate scenarios in clinker production, representing the clinker production process of the Unity Cement Company (UCC) Plant in Yemen.In this study, the ILC's output will be taken into account by; Thermal energy performance (Outlet temperature), Quality (Moister and Ash), and Emissions (NOx, SOx, and CO2).The data was compiled from literatures and the UCC plant.Pre-calciner model was taken from Zhang experiment, which considered the air pollutants emissions [18].The model used in this study is modified to apply the mixture of alternative fuel options as Rahman has suggested before in a similar study [22].Also, it investigated different methods for the equation of state, which are the Redlich-Kwong-Soave (RKS) cubic equation of state and the Boston Matias (BM) alpha function methods (RKS-BM).They are suitable for coal and solid fuel combustion for all thermodynamic properties [17] [23].
Three main points are included in this part to ease the understanding of the process.First, NOx and SOx generation from the pre-calciner will be due to the combustion of the fuel only.Second, CO2 will be produced through the calcination process and combustion of fuels.Finally, only CaCO3 and MgCO3 of raw material will be decomposed within the ILC.
Scenarios examined in this study are A, for the validation and 100% substitution rates, and B, for fuel mixture with various mixture of coal, as follow; [A.1 (Coal 100%), A.2 (X 100%), A.3 (Y 100%), A.4 (X 80% Y 20%), A.5 (X 60% Y 40%), A.6 (X 50% Y 50%), A.7 (X 40% Y 60%), A. Tires/tire-derived fuel (TDF) is the (X) in this study, and plastic waste (PW) is the (Y), both of which are the chosen alternative fuels with coal as the primer fuel.Analysis of X and Y are compiled from the Rahman experiment [17], while the coal analysis from the UCC plant.The results from each scenario are taken from Aspen Plus into Excel Microsoft to analyze them numerically and present the summary in the final tables.The validation data must resemble Zhang's model output.

Validation Scenario
The outcomes from this scenario, Figure 1 displays the process model, are compared with Zhang's [18] simulation results in table 1, and it is within the range of acceptance regarding the UCC plant standards.Figure 1 is also showing the deactivated blocks and streams in order to stabilize the run of the process.The validation scenario showed the possibility of continuing the following methods A. (2-8) 100% substation rate of coal with alternative fuels and B. (1-9) the chosen mixture from the previous scenario with various percentages of coal.

Alternative Fuels 100% Substitution Rate Scenarios
These scenarios were concentrated on the impact of replacing coal with 100% alternative fuels.They were separated into two sections.First, TDF-X and PW-Y were tested individually A. (2 and 3).Second, both were mixed with various substitution rates A. (4-8).The 100% substitution rate "of coal" implementation began with resetting the previous result of scenario A.1 and the deactivation of the decomposition reactor (DECOMP-RYield) and its streams (INBURNER and Q-DECOMP) as seen in Fig. 2. The results of every scenario have summarized in table 2. They only concentrate on the behavior of the main three changes (Energy, Quality, and Emissions) by investigating and analyzing the percentages of (Temperature, moisture, ash, and GHG) as considering the validation scenario realistic and related to the plant 100%.Many factors are the reason for emitting the pollution gases in the environment.In the cement industry, the feed rate, fuel type, temperature in the burning zone, and the amount of airflow are influencing that matter significantly [17].Table 2 shows the investigated emissions were produced immensely in the coal scenario and then decreased within the change of the fuel type scenarios.NOx and SOx reduced enormously because of the small amount of Nitrogen and Sulfur inside the TDF and PW.Also, they only react in the combustion zone, while CO2 emission is yielded by the calcination reactions at (ILC-RSTOIC) and the combustion zone.The scenarios from A. (4-8) notably have decreased their emissions outcomes, which is also related to the substitution rate of PW that has increased constantly.

The Mixture of Alternative Fuels Scenario with Various Substitution Rates of Coal
The naturalistic scenario from A was A.6, which is (50% X, 50% Y) according to the performance indicators (Energy, Quality, and Emissions).The reason for selecting that mixture is due to the stable outcomes and the logical increase of temperature and moisture concurrently.Also, the mitigation percentage of the emitted emissions is considered during the selection and analysis process of the proper mixture.This scenario is selected to participate in Scenarios B with the various percentages of coal.It began with calculating the fuel rate of each scenario, then reactivating the decomposition reactor (DECOMP-RYield) and its streams (INBURNER and Q-DECOMP), as shown in Fig. 3.The inputs of fuel streams are changing due to the investigation of fuel substitution rates.Same as the previous section, the results of every scenario have summarized in table 3.They only concentrate on the behavior of the main three changes (Energy, Quality, and Emissions) by investigating and analyzing the percentages of (Temperature, moisture, ash, and GHG) as considering the validation scenario realistic and related to the plant 100%.This part of observing the environmental impacts evaluation of utilizing the coal substitution rate with the mixture of alternative fuels.Table 3 shows a typical and predictable increase of SOx emissions when the coal percentage is increased, compared with the validation scenario; it is still less at its peak with (-9,93%).
The different part is the unpredictable behavior of NOx and CO2.Table 3 also shows how they start to raise more percentages compared with the validation scenario and then get low.
CO2 has something similar, but it appears earlier than the NOx phenomena.in scenario B.4 (40% coal), it gets (+0,68%), and in B.6 (60% coal) reached its peak with (+1,49%) and then gets low in B.9 (90% coal) with (+0,51%).The reason for such behavior is related to the reaction equilibrium where NOx and CO2 have reached their peaks at different points.

Conclusions
The summary of the process model and its scenarios in this study are present in table 2 for A scenarios, where it is possible to justify the selection of scenario A.6 and table 3 for B scenarios.The choice of the most efficient alternative fuel must have a specific aim to contribute to the production process to the industry demands.
In the case of this study and its results (B scenarios), the best option for the environmental section is less coal and more non-fossil fuels percentage to decrease GHG emissions.Also, the quality section has to be a priority for the clinker production, which means the alternative fuels mustn't contaminate the product and process.Ash analysis must be available to know how far the impact of the product will be, and the moisture amount must be under control in order to keep the burning zone running well.Likewise, Energy is also a necessary core to determining the most efficient alternative fuel option.The outlet temperature is a basis to indicate the best possible alternative fuel connected to similar studies.
The final selection of the most efficient option depends on the policy maker, availability, chemical compositions, and the region itself in the first place.This study can show the possible impacts of the substitution rate of alternative fuel mixtures with and without coal.

Acknowledgement
I would love to thank Professor Hadiyanto, my research supervisor, for his patient instruction, passionate support, and constructive criticisms of this study effort.
I would also want to thank the managers, engineers, and technicians at the Unity Cement Company (UCC) Ltd.Plant-Yemen, for their facilitation and assistance in providing me with the data I needed to run the application.

Table 1 .
Comparisons between simulated results from the UCC plant and Zhang simulation results.

Table 2 .
Summary of A scenarios.

Table 3 .
Summary of A scenarios.