1.1. Motivation, Aim, Gaps in Knowledge and Novelty of the Research

The preceding literature review indicates significant progress in the production and applications of CB. As shown in Table 1, there is a notable gap in investigating methods to improve, utilize and optimise the excess waste heat dumps in CB production processes [59]. Specifically, some studies [48], [54] have examined the exergoeconomic and exergoenvironmental performance of energy systems and revealed the presence of substantial exergy destruction in furnace reaction zones, boiler burners and heat exchangers. Their findings showed that the most significant amount of exergetic destruction took place in the reaction zone of the furnace [60], boiler burners and boiler heat exchangers [61]. However, these studies did not consider the integration of CB production with power plants, which could significantly alter the energy profile economic and environmental outcomes of the CB system [62].

Driven by the need to harness the substantial waste heat energy within the reactor and quench tower streams at the Warri Refining and Petrochemical Company (WRPC) CB plant (CBP) unit in Nigeria, this study was motivated [63]. The research aims to recover and convert the waste heat from CB processing into electric power opportunities. Specifically, it proposes the integration of a STP generation unit into the CB production process. The integrated plant will be modelled and assessed using classical thermodynamic equations, incorporating energy and exergy analyses via Aspen Plus v10 software. At the same time, the techno-economic aspects of this study are examined using the Engineering Equation Solver (EES) software tool.

The novelty of this research lies not only in the unique integration of the plant systems and its comprehensive approach to utilizing exergy, techno-economic and environmental sustainability analyses to examine the integrated CBP and STP production. The objective of this study, therefore, is to identify components and operating parameters that exhibit high exergy destruction and pose environmental risks—areas that have not been thoroughly explored in existing literature for CB production. Additionally, this study introduces the integration of a CB production plant with a STP, leveraging waste heat-to-power generation [64].

2.1. System Description

The WRPC has a CB processing unit, which serves as the focus of this study [63]. The plant utilizes the thermal oxidative decomposition process via the furnace black method and produces ~9000 metric tons annually. The raw material used is conversion oil (decant oil), the composition of which is presented in Table 2. This study also includes an innovative power plant section integrated with the CB unit that recovers and utilizes the enormous heat dumped from the CBP production.

The basic components in a typical CB production plant are the oil preheater, reactor (the heart of the system), bag filter system, pelleting section, drying section, bucket elevator and magnetic separator. The process model reproducibility flowchart and the research methodology flowchart are presented in Appendix Figure A1 and Figure 1. The raw material (decant oil) is supplied through the feedstock section to the oil preheater, where it is heated to about 250°C. The heating is achieved by effluent gases from the reactor, which is fired by natural gas. Temperature control in the preheater is carried out by varying the amount of flow of the oil through the preheater. The preheated oil is sprayed into the reactor. CB is produced in the reactor through partial combustion of the feedstock (decant oil) in inadequate air at a temperature of 1761°C as shown in Equation (1):

The air temperature for combustion is increased to ~55°C using effluent gases from the reactor. The reactor’s smoke passes through a header after the oil preheater, dissipating some heat into the atmosphere before reaching the quench tower. In the study, this dissipated heat is converted into power generation through a configured power plant utilizing the wasted heat, as illustrated in Figure 2(red broken lines).

The quench tower utilizes steam from a steam turbine condenser integrated into the CB production line. CB is continuously separated from effluents through a bag filter system divided into compartments with fibreglass filter bags. The system typically operates at 230–240°C. Effluents enter the upper side of the system, and CB accumulates on bag walls, while clean gas goes into the collection header. Accumulated carbon hampers production, leading to compartment cut-offs. CB is transferred to the pelletizing section using recirculated off-gases through a pneumatic blower. CB is pelleted, dried and passed through a magnetic separator to remove impurities before storage in silos. The integrated plant schematic is illustrated in Figure 2.

2.2. Modelling and Analysis

Aspen Plus v10 simulation software has been used for modelling and developing the plant’s thermodynamic data from each stream of the model. In the stream section, the ideal property of Aspen Plus v10 is selected, which sets the activity coefficient for the liquid phase to 1, the equation of state (EOS) to the ideal gas law, and estimates the molar volume of liquids using the Rackett model. The mixture is considered strongly ideal and contains liquid content with hydrogen, not entirely made of hydrocarbons. In the steam power process section, the IAPWS-95 package is chosen for accurately modelling the thermodynamic properties of water and steam under various conditions, including temperature, pressure and phase (liquid, vapour or gas). These guidelines provide precise calculations for properties such as density, enthalpy, entropy and specific heat capacity of water and steam. The data in Table 3, which is extracted from a standalone STP, are being utilized as input data for modelling the integrated STP in this model. As presented, a mixture of low-pressure methane gas and charged air is combusted in the combustor (COMBUSTR) to release CO-rich combustion gas into the reactor [66], as shown in the model schematic in Appendix Figure A1. Other products of the stoichiometric combustion process from the combustor are CO₂ and H₂O which are produced in the reactor where the heated feedstock is sprayed into the reactor mixture at a temperature of 518°C. A WHR generator is used to recover thermal energy from the reactor outlet in a counter-current heat exchanger to energize cool water to a temperature of 286°C to produce steam for electric power generation via a steam turbine. This recovered thermal energy was used to generate 195 MW of electricity. The CB product is produced in the quench vessel (QUENCH-W) where water at 25°C is added to wash the product prior to arrival at the filters (BACKFILT) compartment where the tail gas in the stream is released for other process utilities. The solid gathered on the walls of the backfilter is produced in the pelletizer (PELLETIZ) unit where additional water is added to drop the stream temperature to about 237°C in a process that causes the CB to develop into pellets. These wet CB pellets are produced in the dryer (DRYER) where a blower is used to discharge hot air over the pellets at 120°C to allow dried pellet production into storage.

2.3. Thermodynamic Assumptions

The design data for the analysis include material flow, operating temperatures and pressures, capacity of components and the material flow type, as well as energy flows where necessary. These input data facilitated the simulation of the system to obtain the thermodynamic properties in the system. The assumptions are categorized as follows:

3.1. Reactor and Steam Boiler Modelling

The exergy at points 5 and 8 is physical exergy, while the exergy at point 8 is both physical and chemical. Accordingly, the exergy at point 7 is represented by Equation (21). Equation (19) calculates the chemical exergy component arising from variations in the concentration of chemical reactions within a gaseous mixture compared to the surrounding environment [88] as presented in Equation (22)

Terms in Equation (22) represent the mole fraction of fuel constituent x_i and the chemical exergy of the fuel constituents, $$. The physical exergy at these points can be obtained using the relationship [89, 90] shown in Equation (23). Equation (24) through Equation (26), which represent the exergy of fuel, product and exergy efficiency, are calculated according to the definition provided by Mohammedi et al. [91].

3.2. Steam Boiler and Quench Tower Modelling

As demonstrated, Equation (29) defines the enthalpy balance in the steam boiler [87]. The enthalpies at stream points 21 and 22 for the working fluid are obtained using Equation (30) or any other property combination. The exergy balance is also written as Equation (31). The terms in Equation (31) represent the exergies at stated points and the exergy destruction, Ex_D,Boiler. The exergy of heat is also represented with Ex_Q. The exergy terms at points 21 and 22 can be calculated using the general relationship in Equation (32) because it is simpler to compute their specific heats at the specified conditions for most state points in the CBP. Conversely, the exergy terms at these points can also be determined using the general syntax provided [94, 95].

In the quench tower model, the effluent gases are sprayed with water to further cool them before reaching to the pelletizing section. The amount of water spray depends on the required temperature drop. Assuming an x% drop in temperature is needed, the resulting temperature of the water and effluent gases mixture is computed using Equation (33). A summary of the exergy balance expressions, exergy of fuel and product for all components is presented in Table 4:

4.1. Purchase of Equipment Cost

The purchase cost of equipment and plant components is expressed as a function of the operating parameters to compute the cost rates associated with plant usage. Expressions for the purchased equipment cost (PEC) are uniquely stated for the components in line with the schematic of Appendix Figure A2. A summary of the cost equations and corresponding auxiliary expressions is shown in Table 5. To evaluate the PEC estimates, the total investment cost of the integrated plant in 1994 was obtained from the WRPC manual [63]. In this work, the current investment cost in 2022 was estimated using the Chemical Engineering Plant Cost Index (CEPCI), where CEPCI in 1994 was 368.1 and CEPCI in 2022 is 816 [102]. The estimated cost of a CBP in Warri in 2022 can, therefore, be evaluated using the following equation: The present cost of the integrated plant is calculated [103] using Equation (40). This approach allows for a comparative assessment of cost escalation over time, reflecting changes in economic conditions and industry standards:

4.2. Plant Exergoeconomic Parameters

The exergoeconomic analysis of the plant is necessary to determine the exergoeconomic parameters of the plant as defined in the equations in Tables 5 and 6. These include the cost of exergy destruction, $$; component exergy efficiency, ψ_k; cost rates of components, $$; component exergy destruction rates, Ex_D,k; and the relative cost difference of the components, $$. The cost of exergy destruction [111] and the relative cost difference [112] are obtained with the expressions in Equations (41) and (42). The remaining exergoeconomic parameters have been discussed in Section 2:

4.3. Cost of Exergy Destruction and Exergetic Improvement Potential

4.4. Environmental Sustainability Indicators

Environmental sustainability indicators are exergy-based indices that comparatively evaluate the performance of energy conversion systems based on exergy efficiency, useful system output and the environmental impact of such systems resulting from thermodynamic irreversibilities. These environmental sustainability indicators are expressed as follows:

Waste exergy ratio (WER): The WER quantifies the degree of cumulative thermodynamic irreversibilities in a plant with respect to the available external exergy input to the system. For thermal power plants and boilers, the available external exergy input is the chemical exergy of the fuel used. The overall waste exergy is determined by combining both the destroyed and the lost exergy within the system [88], as denoted in Equation (45). The WER is obtained mathematically as the overall exergy waste (or destruction) for the system divided by the total exergy input [113], as expressed in Equation (46).

Exergy destruction factor (EDF): The EDF is a crucial parameter that signifies the reduction in the positive impact of the engine on exergy-based sustainability. This factor can be determined by calculating the ratio of exergy destruction to the overall exergy input, as outlined in Midilli, Kucuk, and Dincer [116].

5.1. Thermodynamic Performance and Energy Analysis Indices of the Plant

The performance analysis of the plant is shown in Appendix Table A1. The indices considered were the heat transferred by the components, the quantity of power requirements and the amount of dry carbon produced by the plant. The system has a CB production capacity of 1817 kg/s using a decant oil feedstock of 1870 kg/s and hot flue gas of 346 kg/s combusted in the reactor. The composition of the CB from the simulation shows it has 89.3% carbon, 9.3% hydrogen and 1.4% sulphur (Table 2). Furthermore, it is important to note that the CB process is entirely a low-pressure process occurring at atmospheric pressure. Aromatic oil, which is the feedstock utilized in this model, was initially preheated in storage to about 93°C and later heated at 250°C, generating an exergy flow rate of 128.4 MW to get the aromatic oil to the desirable temperature required to minimize reactor work and allow combustor heat to enter the reactor. Preheater air is also required to enhance the efficient combustion process in the combustor. This blower, operating at 10.17 MW, was used to convert ambient air at 25°C into preheater air at 55°C, further helping the CB system ensure optimal energy utilisation. Due to the high energy content of the flue gas from the reactor, a significant amount of heat released from the CB system has been recovered to power a steam turbine as shown in the T–Q performance chart in Figure 3.

The heat energy released at 518°C was sufficient to produce 662 MW of thermal energy which was recovered through WHR techniques to produce 195.03 MW of electricity for the plant at a steam temperature of 286°C in Figure 3A. The installed pump consumed about 2.34 MW of electricity to deliver 249 kg/s of water at 70 bar to maintain pressure in the steam turbine. A significant pressure drop in the steam turbine occurred to generate electricity, utilizing only about 30% of the 662 MW of thermal energy recovered from the CB reactor. The rest of the thermal energy is dissipated via a condenser into the environment. Without digressing from the focus of this work, it is important to note that the T–Q diagram in Figure 3B aims to provide a clear and concise way to analyse and optimize the heat exchangers (evaporator and condenser) behaviours in the heat recovery process by visually representing the temperature profiles of the working fluids as they transfer heat [70, 117]. As presented in Figure 3B, T₂₁ represents a rising temperature gradient ranging from 27 to 286°C to reach the latent heat absorption point at the steam turbine inlet at a constant temperature, T₂₂. At the turbine discharge, the temperature, T₂₃, drops as the working fluid tends to saturate at the condensation temperature, T₂₀, of 25°C in the pump inlet stream. This T–Q diagram effectively shows the relationship between the temperature gradient of the working fluid and the waste heat transferred from the CBP to the STP plant. It also highlights how waste heat is utilized in the evaporator and rejected in the condenser. This aids in illustrating the heat recovery efficiency, identifying the potential system improvements and optimising the design and operation of the heat exchanger in the STP.

5.2. Exergy Efficiency of the Plant

This work focused on the physical exergy efficiency because it largely considered the efficiency associated with the conversion of mechanical or thermal energy within the integrated plant system. A higher physical exergy efficiency could indicate better performance. The thermal efficiencies of the CB and the STPs were evaluated. It was observed that the CB thermal efficiency was significant at a combustor exergy efficiency of 67.4%. This is an improved plant because, unlike a typical CBP where the combustor heat is dumped to regulate the product specification, in this study, the dumped heat is recovered to energize the power plant, hence improving the thermal efficiency of the CB component of the integrated plant. On the other hand, the STP like most others delivered only 29.12% thermal efficiency, which is also due to over 70% of thermal energy being released to the surrounding at the power plant condenser. The combined plant (CB and the steam power) overall performances were evaluated as standalone as well as overall plant. The overall energy and exergy efficiency of the integrated plant are shown in Figure 4 as 98.75% and 80.40%, respectively.

On the other hand, the energy and exergy efficiencies of the individual plants are also given as CBP (100% and 82.36%) as well as STP (95.61% and 74.34%), respectively. The exergy performance of the individual components of the integrated plant has also been examined, and the component behaviour relative to energy utilization was observed. Other than the exergetic performance of the components of the system, Appendix Table A2 also presents other performance metrics of the plant components including the fuel and product exergies, exergetic destruction, exergetic ratio and improvement potentials of the components of the system.

In Figure 5, the Grassmann diagram illustrates the exergy balance for the integrated CB and STP. The main contributors to exergetic gains are the methane gas combustion (803.35 MW) and the blower air compression (10.17 MW). Significant irreversibilities are associated with the combusted gas cooling across the plant. The rational efficiency of this plant accounts for losses due to irreversibilities, including the adiabatic combustion process (143.53 MW, 17.6%), heat transfer over a finite temperature differential in the evaporator (62.64 MW, 24.1%) and exergy dissipation through tail gas (93.84 MW, 22.9%) and the dryer (75.73 MW, 18.5%). Since the combustor is modelled adiabatically, reducing its irreversibility is challenging. However, irreversibilities related to heat transfer and dissipation can be reduced through effective design improvements and better process integration.

The component with the most exergetic destruction in the integrated plant is the combustor with 261.78 MW, hence the equipment with the most improvement potential. Obviously, the high-temperature gradient from the partial combustion process and the chemical mix of elements during the combustion process are the causes of the large exergetic destruction in the combustor. It is, therefore, important to focus on the differential temperature reduction in the design of the combustor to eliminate the huge destruction of exergy as well as the design of the component to reduce its potential for improvement. The quench water component and the steam turbine are the next most exergetically destroyed component in the system due to the rapid cooling with water at 25°C to the quench water vessel of 350°C and the rapid pressure drop inside the steam turbine from 70 to 0.03 bar. It is observed that the most exergetically destroyed components are also those that have the highest improvement potentials. Figure 6 describes the exergetic destruction of the system in a pie chart pictorial view which documents the percentage destruction of the component exergy relative to the total exergetic destruction of 549.27 MW. It is observed that the combustor alone contributed 48% to the total exergetic destruction of the system. From Figure 6 chart, it is seen that quench water, steam turbine and the evaporator in addition to the combustor contributed 91% of the total exergy these components have destroyed in the plant system.

5.3. Results of Exergoeconomic Analysis

A summary of the exergoeconomic results is shown in this section. The cost streams were obtained and linked to the driving parameters of the system at the component level. The significance of the components in cost formation is depicted through the combination of the component-related cost rate, Ż_k, and the cost of exergy destruction $$. This is equivalent to the total investment cost rate of a component, denoted as $$ presented in Appendix Table A3. External costs to the system were not estimated, and they comprised mainly air and water, as well as CH₄ gas and feedstock oil (decant oil) that were used for firing the combustor and reactor, respectively. These evaluated costs are a function of the irreversibility expressed in these components in Section 3.0. Therefore, Appendix Table A3 details an overview of the economic and exergetic performance of the components of the integrated plant. This exergoeconomic analysis facilitates the understanding of cost formation processes and the equivalent computation of the cost rate of the product and fuel, the determination of the cost implication of exergetic destruction and the cost parameters performance such as the exergoeconomic factor of the integrated plant. The initial capital investment, Ż_k, for the combustor, is uniquely the most significant initial investment with a considerable annualized cost of $1,005,000 and plays a significant role with a cost per hour of $13,317.00. As anticipated, the maximum value of $$ corresponds to the combustor equipped with a burner zone, which has an exergoeconomic factor, f, of 96%, hence, a relative cost difference, ṙ, of 4%. This high $$ value underscores the exergoeconomic significance of this unit, indicating that considerable attention should be given to this component to improve the overall economic performance of the system, as presented in Figure 7. The combustor destroyed exergy cost, $$, is $599.58 per hour, combined with a low exergoeconomic factor, indicating that the cost rate of destroyed exergy is lower than the cost rate of capital investment.

However, it is evident that significant improvements to the combustor performance are infeasible because all the sources of irreversibility, such as combustion, large temperature differences and mixing, are inherent to this unit. The steam turbine with an exergoeconomic factor, f, of 22% and exergetic efficiency of 75% is notably the next highest contributor to the initial capital investment rates and indicates a substantial upfront cost for the crucial component with a high levelized cost per hour at $541.60, emphasizing its impact on hourly operational expenses in the plant. This f value indicates that the purchasing cost of steam turbine is higher than the cost of the destroyed exergy within this component. Therefore, replacing it with a cheaper steam turbine is recommended, even if it comes at the expense of thermodynamic second law efficiency. Table 6 outlines additional exergoeconomic variables that aid in devising strategies for cost reduction. The blower and air heater also incur a high annualized cost that reflects their contribution to the operational expenses of the plant, while the condenser and oil heater demonstrate the opposite; these components have revealed the substantial lower cost of exergy destruction. Hence, there are improved exergetic efficiencies. Thus, these components are the primary contributors to cost formation within the process. On the other hand, an exergoeconomic factor of nearly 1% is observed in both the pump and the quench water vessel, which indicates that the costs of destroyed exergy are significantly close to the capital investment cost. This is further confirmed by the low exergy efficiency of the pump. Improving the exergetic performance of the pump or quench water vessel, even at a higher purchasing cost, can significantly enhance the overall economic performance of the integrated plant.

The cost due to the incorporation of a steam boiler has been accounted for by the evaporator. All cost streams attributed to exergy products are slightly less than the fuel streams and demonstrate the economic viability of the plant. Since the exergy-related costs are directly related to the quantity of exergy destruction, the cost of exergy destruction for the reactor can be affected by employing novel technologies for converting decant oil to CB. A major improvement in the reduction in exergetic cost will be required for the evaporator as it had one of the least f values of 13% compared to the other major components in the CBP. The CB blower demonstrated a high average cost per fuel and product exergy, which indicates significant economic investment, while the near-zero destroyed exergy, Ex_D, and cost rate of exergy destruction, $$, also imply efficient performance. The pump in the STP also demonstrated high costs per fuel, $$, and product exergy, $$, suggestive of significant economic input. However, while the blower shows a high dependency and functionality due to its substantial capital investment, $$, with a high exergy efficiency, ψ, the steam pump, on the other hand, displays moderate exergetic destruction with associated extremely low capital investment as well as low exergy efficiency that needs to be improved. The oil preheater, valve, air heater and condenser all indicate moderate to negligible costs per fuel and product exergy, which indicates a cost-effective operation. They express a minimum cost scenario due to small-to-moderate exergetic destruction in the components. However, while the valve and the air heater both show indications of high capital investment, the oil preheater and the condenser demonstrate some extremely low capital investment. As shown in Table 7 and Figure 7, it should be noted that improving the performance of individual components may have a deteriorating impact on the overall exergetic and exergoeconomic performance of each component. Therefore, recommendations to enhance the economic performance of each component should be achieved through comprehensive component design. Attempting to improve individual components does not necessarily lead to enhanced performance for the entire integrated plant system. A summary of initial investment (PEC) and levelized capital cost rate are calculated by converting CEPCI 1994 cost (in USD) to 2022 (in USD) for the components of the system, as shown in Table 7. The cost analysis was done using the EES platform. The high points indicated that the ST turbine and the combustor both contribute about 91% of the total cost attributed to the plant, with the ST turbine and combustor associated with 73% and 18%, respectively, to the investment cost. Though having a relatively low cost per unit of energy, it contributes significantly to the overall cost, indicating its critical role in the plant.

However, efficient resource allocation that prioritizes investments in components such as the steam turbine and combustor will be required due to their significant impact on both initial and ongoing operational costs. Strategic planning, investment prioritization and potential optimizations to ensure plant financial performance over its operational life would be helpful in reducing the overall project cost over its life cycle. It is also indicated in Table 7 that most components presented a moderate cost per fuel and product exergy, suggesting a considerable economic input and substantial cost rate of exergy destruction, that is, combustor, quench water vessel and the steam turbine. The component with moderate cost rates of exergetic destruction (reactor, back filter and dryer) and the low exergy destruction with associated low-cost rates (evaporator and pelletizer) presented various degrees of capital investment cost as well as low moderate or high component investment rates which could indicate no, little or high potential for improvement. In summary, the plant exergoeconomic analysis highlighted the efficiency, economic input and potential areas for improvement in each component’s operation. The values in each column offer insights into the overall exergoeconomic dynamics of the plant.

5.4. Results of Environmental Sustainability Analysis

This analysis evaluates the environmental impact of the integrated plant to ensure that this model meets today’s needs and does not compromise the capabilities needed for the future by understanding the environmental aspects and potential consequences associated with the operation of this plant. The exergy-based environmental sustainability analysis is presented in Table 8. As widely recognized, CB tail gas stands out as the primary by-product in the CB industry, constituting ~30% to 40% of the total energy expended in CB production. This tail gas is a notable source of air pollution, characterized by its low calorific value and polluting nature. Harnessing the full potential of CB tail gas not only presents an opportunity to conserve energy and minimize consumption but also offers a means to mitigate pollution in the industry.

This paper presents the exergetic assessment and exergetic sustainability parameters of the CBP and the integrated CBP and STP configurations, otherwise tagged as CBP + STP as shown in Table 8. Prerequisite to determining sustainability parameters of these plants, the exergy parameters of the components needed to have been determined as presented. Furthermore, to compare the different energy systems based on their efficiency and environmental footprint, the ESI matrix is employed in the selection of more sustainable options. Exergetic sustainability results are presented in Figure 8 and Table 8 to understand how each of the sustainability indicators including, WER, exergy efficiency, EDF, EEF and ESI affects each of the plant configurations. It is observed that the STP inclusion in the CB cycle slightly increased exergy efficiency from about 74.3% to 80.4%, whereas a decrease in exergy efficiency is observed in the CBP by 2.4%.

The improvement in the STP evidently showed an 8.2% cascaded improvement in the EDF. However, there is a fractional increase in the waste–exergy ratio of about 13%, which is twice the rate of exergetic efficiency increase. This indicates that the exergetic losses or waste from the CBP are not recovered. Otherwise, most of the tail gas that constituted this waste could have been used as fuel in the STP or for other district heating purposes. However, due to the power contribution from the steam turbine, the system’s ESI decreased by 0.627 units, which is about an 18% increase from a very high sustainability index value of 4.106. It is important to note that the exergetic sustainability values are desired to be above unity. However, the decrease in the sustainability index with the integrated plant is due to the enormous thermal energy released by the condenser. Otherwise, the integrated system had the capacity to increase the ESI by further utilising the condensate dissipated heat streams for other needs, including district water heating and bottoming ORCs. As shown in Figure 8, the CBP clearly showed substantial evidence of environmental impact, while the CBT + STP has shown and contributed a much-reduced impact. Therefore, the integration of the STP in the CBP is worthwhile for increasing the plant exergetic efficiency and WER, as well as being effective in overall ESI improvement.

The high ESI in Table 8 confirms the presence of emissions in the integrated plant. Most prevalent are the emissions in the tail gas released from the bag filter consisting primarily of CO, CH₄ and NO_x compounds, which are comparable to the impurity levels in flue gases from coal-fired power plants [118]. Carbon dioxide is the primary GHG on earth, absorbing and radiating heat from the planet’s surface. CO₂ accounts for two-thirds of the total warming influence of all human-produced GHGs [119]. The emission factors for carbon dioxide from CB production in the literature range from 1.90 to 5.25 kg of CO₂ per kg of CB (kgCO₂ per kgCB) [120]. These estimates encompass a wide variety of plant configurations, feedstock compositions and product types. Emissions from the CB production process originate from the combustion of decant oil and natural gas used in the reactor. This process produces a tail gas, which can be utilised to generate steam for electricity or sold. The composition of these emissive gases varies depending on the grade of CB manufactured.

Table 9 compares the performance parameters of this integrated plant with those of similar and conventional plants reported in the literature. It is observed that the parameters of this work are consistent with the conventional data and perhaps most data in the literature for each key component of the CBP system. More importantly, the table compares the molar concentration of the tail gas composition in this work to those found in the literature. However, only about 0.01% of CO₂ is contained in the tail gas produced compared to the conventional 1.5–3.9 mol concentration in the literature. The low carbon concentration in this model may be due to the excess 20% air used in methane gas combustion in the reactor. To minimize emissions in a CB production process, several strategies can be employed. This may include the use of cleaner fuels such as natural gas instead of furnace oil and the implementation of advanced combustion technologies that ensure more complete fuel burning, as well as optimizing process efficiency, such as improving heat recovery systems and reducing energy losses, all of which processes have been adopted to lower overall emissions in this work. However, the integration of carbon capture and storage (CCS) and the utilization of the tail gas produced during the process to generate steam and electricity can both reduce the environmental impact, but unfortunately, this could be the objective of another future study.