1. Introduction
Integrated gasification combined cycles (IGCC) have received increased interest over the past few years as a more sustainable solution for the co-generation of power, steam, and other valuable by-products from heavy refinery residues. The high flexibility of IGCC systems on the input feed makes them very attractive for employment in various applications subjected to fuel availability and costs. In this way, a wide range of solid and liquid wastes can be used as base input fuel, including coal, municipal wastes, heavy liquid refinery residues (heavy oil), solid refinery waste (petroleum coke), and biomass, among others [
1]. IGCC power systems provide improved economic viability to existing and new processing plants owing to their ability to generate power from syngas and additional high-valued utilities and feedstock chemical co-products (e.g., ammonia, methanol, etc.) from low-cost fuels [
2,
3]. Additional advantages include low production costs and the potential to meet tighter pollutant emissions standards on SO
x, NO
x, and particulates. IGCC systems also offer the possibility of reducing CO
2 emissions via pre-combustion gas purification, carbon capture and storage (CCS) [
4], and gas switching combustion (GSC) technology [
5]. Even though recent advances in gasification, syngas cleanup, air separation, and gas turbine technologies have driven process costs down and increased system performance, challenges remain in further enhancing energy and environmental performance requirements. In this context, the integration of renewable energy (particularly solar energy) into IGCC systems emerges as an attractive alternative.
IGCC power systems are composed of three main sections, namely, gasification, clean-up, and combined cycle [
6]. The construction, operation, and maintenance of IGCC plants are generally more demanding than conventional power systems, eliciting an increase in capital investment and maintenance expenses and a reduction in reliability and availability [
7]. Typically, IGCCs allow obtaining higher thermal efficiency over conventional coal-fired power plants (up to 60% when fueled with natural gas [
2,
8]), while the carbon emissions are lower than traditional power plants [
7]. Another important benefit of IGCCs is the reduction of other pollutants. This is because the cycle allows the syngas to be purified after leaving the gasification reactor while a substantial amount of hydrogen sulphide and carbon dioxide is separated during the process [
9]. In some cycles, the latter can be done via humidification, with the advantage of optimizing the amount of hydrogen produced in the syngas stream [
10]. In an IGCC plant, oxygen blow technology can also be used in the gasification reactor to prevent nitrogen gas from interfering in the gasification process, by prompting a binding effect in reducing the toxic NO
x pollutant [
11].
Nag et al. [
12] performed energy and exergy analyses of a combined cycle with a fixed bed gasifier. Their results showed that the gasification reactor unit displayed the highest energy loss among other cycle components. Moreover, their exergy analysis results revealed that the temperature ratio did not influence the highest exergy destruction of the gasification process, whereas the pressure ratio had also a low effect. The authors concluded that the exergy destruction in the combustor can be reduced by increasing temperature and pressure ratios. Emun et al. [
13] simulated an IGCC system employing a Texaco gasifier using Aspen Plus
® software. The system was operated via gasification using slurry based on a coal–water mixture (35.5%
w/
w water). Their results showed that the proposed process can achieve up to 45% thermal efficiency with a substantial decrement in SO
x and CO
2 emissions, reaching 0.15 kg/MWh and 698 kg/MWh, respectively.
Domenichini et al. [
14] investigated hydrogen and power generation through a system fueled by refinery waste via pinch analysis. Their proposed approach was based on oxygen blow-entrained bed gasification, which was sized to produce a considerable hydrogen amount and feed gas turbines of the combined cycle unit. The authors focused on the heat integration among syngas cooling and sections of the combined cycle and reported a cost of 34 €/t for avoiding CO
2 emissions. Additionally, they concluded that the cost of electricity was attractive, with a hydrogen price of 9.5 cents/Nm
3, highlighting the advantage of the combined production. Morini et al. [
15] simulated an IGCC cycle with a 650-MW gasification unit. In addition, the authors considered an air-cooled driven gas turbine, in which the inlet air cooling was achieved using liquid nitrogen spray. They also accounted for actual temperature profiles of various locations for an entire year in the modelling formulation. Their findings indicated that the system presented the highest performance for Johannesburg, when compared to conventional inlet air cooling schemes.
Zhang and Ahn [
16] investigated the prospects of two IGCCs in China for achieving near-zero emissions. Their proposed system was based on a pre-combustion gasification process with carbon dioxide adsorbent and sour and sweet shifts. The authors reported that the sweet shift required 4.6-fold more shift steam than the sour one due to the low amount of steam in the syngas after the removal of H
2S. Khoshgoftar Manesh and Jadidi [
17] assessed the performance of a biomass-based IGCC fueled with olive pits by applying energy, exergy, exergy-economic, and exergy-environmental (4E) analyses. Their results indicated an output power of the IGCC of approximately 387.3 MW, with a cost of 5.23 US
$/s and pollutants emissions of 41.72 mpts/s. Moreover, their results demonstrated that the gasifier had the largest exergy destruction rate, which accounted for a 24% share. The authors also showed that the gasifier and combustion chamber units had the highest avoidable exergy destructions, while the scrubber presented the lowest one. Szima et al. [
5] implemented a techno-economic analysis of hydrogen and power production from the IGCC plant. The authors employed the gas switching combustion (GSC) for carbon capture, whereas membrane-assisted water gas shift (MAWGS) reactors were used for hydrogen production.
Table 1 displays a brief overview of the most recent literature regarding applications, methodologies, and practices related to IGCCs.
Although previous studies constitute important contributions to the area, none considered energy, environmental, and economic analyses of integrated solar-aided gasification cycles for enhancing energy recovery from low-cost opportunity fuels. It should be noted that solar-aided IGCC systems are among the most efficient and cleanest methods for converting solid wastes and refinery residues into valuable power and heat utilities and other high-value chemical by-products needed by the refineries. For surpassing limitations on preceding research, this paper investigates a new integrated solar-assisted gasification cycle (ISGC) for producing steam and power from heavy refinery fuels while enhancing energy efficiency, economic, and environmental performance indicators. In the proposed cycle, the syngas from the gasifier enters the turbine to produce power after purification and separation of pollutants, and then it is sent to the steam section for thermal energy recovery. In addition, a solar energy field, a two-pressure heat recovery steam generator (HRSG), and CO2 capture unit are integrated into the plant to enhance the overall system efficiency while reducing carbon emissions. The ISGC system was modelled and simulated in MATLAB software, and the results were validated using Thermoflex software. Conventional and advanced 4E analyses were implemented to assess energy, economic, and environmental ISGC system performances. The 4E analyses allows examining system units with an increased level of detail and thereby prioritize these equipment pieces for future optimization and improvement. Hence, the new approach provides a novel avenue towards dealing with heavy refinery fuels and turning an environmental issue into an opportunity, especially for oil-rich countries that enjoy high solar global irradiances. Major contributions and innovative features introduced by this study are highlighted as follows.
- (1)
Combined heat and power co-generation from gasification of low-cost heavy refinery fuels for achieving enhanced energy recovery.
- (2)
Solar-assisted energy generation via parabolic trough collectors to improve the overall system efficiency while reducing environmental impacts.
- (3)
Integration of CO2 absorber technology for further reducing pollutant emissions.
- (4)
Comprehensive conventional and advanced energy, exergy, exergoeconomic, and exergoenvironmental analyses to evaluate main performance parameters and identify potential system improvements.
2. System Description
The integrated solar-gasification cycle is based on the IGCC cycles proposed by Zhang et al. [
29] and Khoshgoftar Manesh and Jadidi [
17], which provide a suitable base combined-cycle configuration. Yet, the proposed new ISGC system encompasses a partial gasification cycle under oxygen blowing with a pre-combustion chamber, together with a solar energy field (parabolic through collectors), two-pressure heat recovery steam generator (HRSG) and a CO
2 absorber, as depicted in
Figure 1. It should also be noted that the study by Zhang et al. [
29] used coal, and Khoshgoftar Manesh and Jadidi [
17] employed biomass as feed for the IGCC power system. In contrast, the proposed ISGC plant is targeted at using heavy refinery fuel to take advantage of low-cost heavy refinery residues. Furthermore, conventional and advanced exergy, exergoeconomic, and exergoenvironmental evaluation are implemented to better shed light on the ISGC power system.
Table A1 provides the operating conditions assumed for the different ISGC system units.
In the proposed ISGC system, the air first goes to the air separation unit (ASU), where the pure oxygen product stream (5) enters the gasifier. The superheated water vapor by the solar field is mixed with the heavy refinery fuel stream before entering the gasifier. In the syngas cooler, the heat from the syngas stream is utilized to saturate the water in the steam turbine. It should be observed that the fuel and syngas streams are represented by black lines in the schematic diagram in
Figure 1, whilst the water and steam streams are depicted in blue, the flue products in red, and the air stream by red lines. The syngas enters the heat exchanger as stream (6), which is responsible for increasing the temperature of the humidified syngas. In the next section, the syngas goes to the H
2S remover unit in stream (10). Then, the syngas stream enters this unit as stream (4) to capture CO
2. After entering the humidification section, the temperature of the syngas in the mentioned heat exchanger increases. Thereafter, the syngas pressure needs to be synchronized with the compressor outlet pressure. Hence, the syngas pressure is reduced by half of its initial value via an expander and produced energy. Herein, the syngas entered the combustion chamber (CC) as stream number (16). Finally, the combustion gases are sent to the gas turbine (GT) to provide electrical power.
In the HRSG unit, water enters the first cycle economizer as stream (42), and, after being saturated in the evaporator as stream (32), it goes to the first superheater to exchange heat with the flue gases to become superheated. Afterwards, this superheated steam enters the first turbine as stream (33). An additional water stream (1) entered the second economizer, before entering the syngas cooler as stream (11) to change phase from saturated liquid to saturated steam. It is then mixed with stream (27) and enters the second superheater. Here, superheated steam is sent to the second steam turbine (ST) with the stream (38). After power generation, this stream goes to the condenser, and, after condensation, it finally leaves it as a saturated liquid stream (22).
4. Results and Discussion
The results obtained from the conventional and advanced thermodynamics, exergy, exergoeconomic, and exergoenvironmental analyses are presented in the following sections.
4.1. Validation of Thermodynamic Modelling Results
This section compares the modelling results for mass flow rate, temperature, and pressure of process streams obtained from MATLAB to those from the simulations performed in Thermoflex software, which are based on real plant data information. The results comparison is shown in
Table 2. For all streams, the simulated results showed an error lower than 1%, presenting an appropriate agreement with each other.
Table 3 displays the output parameters for the power and heat transfer rates of the different system components obtained from Thermoflex (based on real plant data information) and MATLAB simulations. In all cases, the simulated results presented an error lower than 4.5% and, therefore, an appropriate agreement with each other.
Table 4 exhibits the main results of the proposed ISGC modelling approach compared to those found by Zhang et al. [
29], including the exergy input to the system, exergy destruction, net output power, thermal efficiency, and the ratio of exergy destruction to total input exergy. The results showed a similar thermal efficiency of around 50% for both studies. Even though the pre-combustion chamber was omitted in the present system (compared to Zhang et al. [
29]), the ratio of exergy destruction to total input exergy was decreased by 4.8% (from 56.7% to 53.98%). Therefore, in the output power scale of the cycle, this configuration also reduced the investment cost. It should be noted that the main goal in the study was to increase exergoeconomic efficiency; therefore, expressing the increase in thermal efficiency from 49.5% to 50% indicated that replacing the coal fuel feed with heavy refinery fuel in cycle did not reduce the relative thermal efficiency but also caused a slight improvement. Furthermore, the system produced 319.92 MW of power by feeding on 15.5 kg/s of heavy refinery fuel. This occurred at a thermal efficiency of 50.01% and exergy efficiency of 53.98%.
4.2. Conventional Exergy-Based Analyses
Table 5 shows the results obtained for the conventional exergy, exergoeconomic, and exergoenvironmental analyses for each process stream through system simulations using MATLAB software environment.
Table 6 displays the exergy, exergoeconomic, and exergoenvironmental analyses’ results for each equipment piece in the cycle. The results show that the exergy efficiency of the solar field and the condenser was approximately 20% and 43%, respectively. One reason for such exergy efficiency values is the relatively low efficiency in the air separation unit (38%). The latter value was owed to using a membrane-based oxygen separation system, which required high-power-consuming compressors.
In addition, the results revealed that the combustion chamber had the highest exergy destruction cost. The combustion chamber also presented higher cost and environmental impacts of the exergy destruction when compared to other equipment units. Therefore, its performance should be improved to mitigate its exergy destruction. Heat recovery equipment had the lowest exergoenvironmental coefficient, which indicated that the exergy destruction of this equipment had high adverse effects on the environment. This is affected by thermodynamic factors and other reasons, for instance, the equipment weight, which means that it should be optimally designed from thermodynamic and structure to prevent adverse environmental effects. A practical suggestion to reduce these effects in the heat recovery equipment could be to couple them to a solar energy field to increase the temperature of the water entering the system. In the investment cost section, the highest amount was related to the gas turbines, and the lowest was presented by the condenser.
To better understand the critical parameters of the system, the share of exergy destruction in the system components is depicted in
Figure 2. Hence, the combustion chamber, the gasifier, and the HRSG unit had the highest share of exergy destruction. The rate of exergy destruction in the combustion chamber was 43%, which can be reduced by implementing an optimal design approach. Compared to other similar cycles, a noteworthy point in the present system is the relatively low exergy destruction share of the gasifier (25%), which was due to the employment of the air separation unit and, thereby, the injection of pure oxygen.
Figure 3 displays the cost flow diagram of the proposed system. As observed, the amount of exergoeconomic destruction of the combustion chamber, HRSG, and gasifier was higher than other equipment. Moreover, in terms of equipment cost rate, the highest cost was related to the gasifier, followed by the gas turbine. The remarkable point in this figure that distinguishes this system from other conventional cycles is that the total exergy destruction costs were lower than the total equipment costs. This is owed to the inexpensiveness of the fuel employed in this system, which was a direct function of the exergoeconomic destruction of the equipment.
The total cost of exergy destruction was equal to 164.27 US$/min, whereby the highest rate was related to the combustion chamber with 46.17% share, followed by the HRSG with 17.41%. Besides, in the equipment cost rate section, out of the total 188.52 $/min, the highest, with 43.46%, was related to the gasifier, followed by the purifying section at 9.05%. The sum of these costs caused the rate of total cycle exergy destruction at 446.02 $/min.
The system environmental impacts, including the environmental impact rate of the equipment, exergy destruction, and the fuel entering the cycle, are displayed in
Figure 4. The summation of these effects led to the total environmental impact rate of the system. As observed in this figure, the environmental impact rate of exergy destruction of the combustion chamber, HRSG, and the gasifier was higher when compared to other equipment pieces in the system. It is noteworthy that the total environmental impact rate of exergy destruction of each piece of equipment was less than the environmental impact rates of the equipment. The reason behind this is the high calorific value of the fuel used in this system, which was a direct function of the exergoenvironmental exergy destruction of the equipment.
The total rate of environmental exergy destruction effects was 70,961 pts/min, in which the highest rate was related to the combustion chamber, with a 37.67% share, followed by the HRSG unit, with 24.02%. The total equipment environmental impact rate amounted to 22.2 pts/min, among which the HRSG had the highest cost rate, with a 30.75% share, followed by the gasifier, with 17.2%. The sum of these environmental effects led to the total cycle environmental impact rate of 72,796 pts/min.
4.3. Advanced Exergy-Dased Analyses
Figure 5 displays the advanced exergy analysis, namely, avoidable, unavoidable, endogenous, and exogenous exergy destruction in each system equipment. The highest avoidable exergy destruction was related to the condenser, followed by the syngas cooler and the gas turbine. Moreover, the highest endogenous exergy destruction was associated with the solar field, followed by the combustion chamber and the CO
2 capture unit. To increase the exergy efficiency, system components with high avoidable exergy destruction such as the condenser, syngas heat exchanger, and gas turbine can be prioritized for modifications. Based on avoidable exergy destruction, the steam turbines 1 and 2, air compressor, and CO
2 capture unit had the minimum potential for improvement. Hence, these components do not require any modifications or improvement. In addition, steam turbines 1 and 2 and condenser had minimum endogenous exergy destruction.
Figure 6 depicts the advanced exergoeconomic destruction share of each system equipment. As shown, the condenser also had the highest avoidable exergoeconomic destruction share, while the steam turbines had the lowest. Therefore, components with high avoidable exergy destruction such as the condenser can be prioritized for modification to increase the exergoeconomic efficiency. This modification can include changing the type of equipment used, regardless of the cost of exergy destruction. In addition, the syngas cooler, gas turbine, economizer, scrubber, and CO
2 capture are in the following priority for improvement. However, steam turbines 1 and 2 presented minimum potential for improvement based on the avoidable parameter. Based on the endogenous parts of the cost of exergy destruction, the combustion chamber, gas turbine, air separation unit, H
2S removal, and CO
2 capture had the highest rates.
The advanced exergoenvironmental destruction share of equipment is shown in
Figure 7. Similarly to the previous results, the condenser had the highest avoidable exergoenvironmental destruction share among the equipment units. Moreover, it is noticeable that the components unavoidable exergoenvironmental destruction share in all equipment was higher than avoidable (except for the condenser). In addition, components with high avoidable exergy destruction such as the condenser, syngas cooler, Economizer 2, and GT can be prioritized for modification to increase the exergoenvironmental efficiency. In this case, this modification can be grounded on the type of equipment used in terms of manufacturing processes and lower weight.
Table 7 presents the ranking of the impact of exergy, exergoeconomic, and exergoenvironmental destruction rate parameters in the conventional and advanced analyses for each equipment used in the ISGC system. This table helps determine the distribution and extent of the maximum exergy destructions and identify the effects in the cost and environmental sections to minimize the exergy losses in the optimization step in terms of the least amount of time and cost.
As shown in the conventional results in
Table 7, the highest exergy destruction was related to the CO
2 capture unit, while the lowest rate was associated with the syngas cooler and combustion chamber, simultaneously. Regarding the exergoeconomic destruction rate, the highest value was associated with the solar field (20 MW) and the lowest with the combustion chamber (1 MW). For the sum of exergoeconomic destruction and investment cost, the highest amount was related to the expander (20 MW), whereas the lowest one was to the gas turbine (1 MW). For exergoenvironmental impact rate, the solar field had the highest value (20 MW), while the combustion chamber (1 MW) presented the lowest. Finally, for the sum of component and operational exergoenvironmental impact rate, the highest and lowest values were pertinent to the combustion chamber and solar field, respectively. The highest and lowest values herein indicate the strengths and weaknesses of the present cycle that can be used for optimization and future work, where equipment pieces with lower exergy efficiency can be the focal point for future optimization analysis.
In the advanced analysis section, the highest amount of avoidable exergy destruction was associated with the steam turbine 2 (20 MW), while the lowest was attributed to the condenser (1 MW). The steam turbine 1 (20 MW) had the highest endogenous exergy destruction, whereas the solar energy field showed the lowest one. In addition, the highest rate of endogenous exergoeconomic destruction was related to the steam turbine 2 (20 MW), whereas the combustion chamber (1 MW) presented the lowest rate. The highest avoidable exergoenvironmental impact rate was related to the steam turbine 2 (20 MW), while the lowest was associated with the condenser (1 MW). Finally, the highest rate of endogenous exergoenvironmental impact rate was related to the steam turbine 2 (20 MW), whereas the lowest one was to the combustion chamber.
5. Conclusions
This paper investigates a new integrated solar-assisted gasification cycle (ISGC) for producing steam and power utilities from low-cost heavy refinery fuels while enhancing energy efficiency and economic and environmental performance indicators. In the proposed ISGC system, syngas from the gasifier enters the turbine to produce power after purification and separation of pollutants. Then, it is sent to the steam section for thermal energy recovery. In addition, a solar energy field and a two-pressure heat recovery steam generator (HRSG) are integrated into the plant to enhance the overall economic and environmental plant viability. In this new study, the ISGC system was modelled in MATLAB software, and the results were validated using Thermoflex software. Conventional and advanced energy, exergy, exergy-economic, and exergy-environmental (4E) analyses were implemented to evaluate the economic and environmental system performance. In addition, the 4E analyses allowed for the detailed examination of the different system units and the determination of a prioritization order for future optimization and improvement.
The results obtained from the 4E analyses revealed that replacing the coal fuel feed with heavy refinery fuel allows for a slight improvement in the thermal efficiency, from 49.5% to 50%. Moreover, the ratio of exergy destruction to total input exergy was decreased by 4.8% compared to results for the conventional cycle in the literature. The latter result indicated that the proposed ISGC configuration can also reduce the plant investment cost. Besides, the system produced 319.92 MW of power by feeding on 15.5 kg/s of heavy refinery fuel, at a thermal efficiency of 50.01% and exergy efficiency of 53.98%. The results also showed an investment cost of $466 million, gauged at a system cost rate of 446 $/min and an environmental impact rate of 72,796 pts/min. To sum up, these results demonstrate that the proposed ISGC is theoretically viable for converting heavy refinery fuels into syngas and producing power. Therefore, the new approach provides a feasible solution towards reducing environmental impacts related to heavy refinery fuels, particularly for oil-rich countries with high availability of solar resources.
The thermodynamic simulation results show that the computer code developed in MATLAB has high accuracy compared with the Thermoflex software (with overall errors lower than 4.5%). This verification of results is acceptable for steady-state conditions, and high-accuracy is achieved. However, for dynamic and transient analyses, these assumptions are not enough. In this case, the kinetic model must be considered for the gasifier. Additionally, part-load conditions for turbines and compressors should be implemented.
Some recommendations for future research include adding a set of solar collectors to the entrance of the HRSG unit for preheating the inlet water, which could further improve energy efficiency; using pure oxygen produced in the high-pressure air separation unit to feed the combustion chamber (instead of atmospheric air) while eliminating the air compressor since this alternative could lead to higher combustion efficiency and a decrease in investment costs; using different carbon capture technologies; optimizing main design and operating parameters; using a gasifier system with higher temperature and pressure tolerance; and, finally, comprehensive footprint assessment can be performed for improving environmental analysis of a proposed system.