Previous work

The first works presented by the bibliographic analysis are those that address the issue of emissions in the refinery, they will focus on sources and then study a possible carbon dioxide technology. I will then move on to publications concerning the analysis of energy flows.

Only in recent years has there been interest in a possible implementation of CO2 capture in refineries. In the past, many works have studied the feasibility and underlined the synergies of this process with coal-fired power plants or with iron and steel industries or paper mills because they require large amounts of heat and at the same time have high waste heat that can be reused. This interest is due to climate change and the increase in the cost of oil, in fact when it increases, everyone try to minimize waste to avoid extra costs.

The choice of absorbent medium is very important. In the study done by Andersson, Jilvero, Franck, Normann and Berntsson described in the article "Efficient Utilization of Industrial Excess Heat for Post-combustion CO2 Capture: An Oil Refinery Sector Case Study" it emerges that for the same heat recovery, if I compare ammonia and MEA as solvents, the MEA is able to absorb more CO2 and consequently the plant has a higher efficiency. In the article, the MEA is taken as a reference as it is one of the most used solvent and is considered the reference point for the study of post-combustion capture. It also has a low regeneration temperature, which is very important in the selection as there is often a lot of heat available but at a low temperature, which is not enough to be reused. The idea of using NH3 comes from studies that estimated the heat required for regeneration to be approximately 2500 kJ/kg CO2 captured; the MEA needs 3300 - 3800 kJ/kg CO2 captured. However, an important parameter is to evaluate the operating conditions of the stripper: temperature and pressure determine the necessary heat. In the study of the model three temperatures are taken into consideration for the stripper: for the MEA 90, 105 and 120 °C; for ammonia 105, 120, 135 and 155 °C. It is not possible to work at 90 °C for NH3 due to the extensive slip. Both processes are studied on the same absorber/stripper setup. However, NH3 is more volatile than MEA and therefore a second absorber is required. There is also a heat requirement for the NH3 stripper that must be taken into consideration. The processes are shown in the Figure 3.1.

In this paper, a refinery in Sweden is studied, which has a demand for 409 thermal MW; the emission points are four chimneys with a CO2 concentration that varies. From a detailed study, it emerges that the real heat consumption is 199 MW. Of this heat, it is estimated that

Figure 3.1: General setup of the MEA/NH3 absorption processes. Equipment within dashed lines is only used in the NH3

process. [19]

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only 20% can be recovered (42 MW). Heat is available from 130 to 170 °C. The results of the work of Andersson, Jilvero, Franck, Normann and Berntsson are shown in the Figure 3.2 and what emerges is that as the stripper regeneration temperature decreases, the heat demand increases. In the case of low temperatures, NH3 is present in the captured CO2 and therefore must be removed before compression. On the one hand there is MEA which suffers from the evaporation of water, which is always present with the solvent, while on the other hand there is the ammonia which suffers from loss of absorbant.

Table 3.1: How much CO2 can be captured using only excess heat, and how much that will be emitted to the atmosphere (rem=remain) or to be captured using primary heat.

Temperature

Figure 3.2: Heat demands from the CCS capture plant before (Current situation) and after (Future situation) energy efficiency measures are being implemented. [19]

The results presented in Table 3.1 give the somewhat paradoxical conclusion that energy efficiency combined with CCS in this case actually would result in larger CO2 emissions than only performing CCS. The current situation represents the actual one, while the future one occurs through an optimization of the heat exchangers and allows to save 5% of the heat compared to the current case. Only if operating with MEA at a stripper temperature of 90 °C do the CO2 emissions decrease more for the combined measures. For the system presented in this case study a MEA process operated at a stripper temperature of 90 °C was shown to have the smallest need for supplementary heating when capturing 85% of the CO2 in the flue gases. However, modeling a stripper to work at this temperature is not easy and always presents uncertainties.

Gardarsdottir, Normann, Andersson and Johnsson study the implementation of post-combustion CO2 capture technology in the paper "Process evaluation of CO2 capture in three industrial case studies" in three different realities. The work will identify the emissions of three industries and the available waste heat. The authors will compare pulp and paper industry, (petro) chemicals industry, and ferrous and non-ferrous metal plants and then compare the results with a coal-fired power plant, which represents a benchmark for CO2

capture technology because is studied in many research works. It must be borne in mind that it is important to study CO2 capture technology also for many industries as it is often not possible to reduce the production of carbon dioxide. Again, MEA and ammonia are used as they represent well known bases for post-combustion processes. The Figure 3.3 summarizes the results of the work. In the case of the pulp mill there is no possibility to use the waste heat because it is of low quality. The necessary heat was calculated based on obtaining an 85% capture efficiency.

Figure 3.3: Available waste heat as a share of the heat demand for capture and CO2 capture rate with waste heat alone. [2]

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In the case of the power plant, the heat demand is equal to 0 as it is taken as a reference and has not been studied. In the case of the aluminum factory, the waste heat that can be recovered is almost 60% of the total; for the refinery, on the other hand, about 25% can be reused. In the case of the refinery, the hydrogen production process has been studied because the gases produced by this process have the highest CO2 content (about 24% mol).

In the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through process simulation" a precise refinery process is studied to be combined with carbon capture technology. Fluid catalytic cracking is one of the most important processes in the refinery as it allows the transformation of heavy hydrocarbons into lighter molecules such as gasoline and diesel. However, it is also a process that emits a lot of CO2 emissions and to solve the problem, carbon capture can be used. Obviously, the cost of producing heat for solvent regeneration affects the refinery economy. In this paper, the authors try to study a possible heat integration by analyzing different heat recovery sources. The flue gases of the FCC process have a critical temperature of 600 °C which can be exploited. The flue gases must be treated before entering the CO2 removal plant. The base case studied is shown in the Figure 3.4. The gases enter a turbine to recover energy and subsequently into a waste heat steam generator (WHSG) where the heat is recovered to produce steam.

The gases leaving the boiler are still at a high temperature and can be used for the regeneration of the MEA in the reboiler of the stripper. Three possible cases are analyzed:

1. Only FCC excess heat are supplied to the CO2 capture process: in this case the heat required by the CCS is entirely provided by the excess heat of the flue gases. It is considered that the electrical energy has a higher value than the thermal energy and therefore a heat exchanger is added after the WHSG.

2. Guarantee 90% CO2 capture level with FCC unit excess heat only: the diagram of the system is shown in the Figure 3.5. To achieve 90% capture efficiency, it is necessary to give up some of the work produced by the turbine. The gases leave the turbine at higher temperature than in case 1.

Figure 3.4: Process diagram of the industrial FCC unit (reference case of the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through process simulation"). [21]

3. Guarantee 90% CO2 capture level with additional heat supply: extra heat supplied by the steam network must be used to leave the energy produced by the gas turbine unaltered.

The Table 3.2 summarizes the results of the cases analyzed by the paper. In case 1, it is possible to obtain an efficiency of 78% only by cooling the gases that would still be wasted.

However, the number is far from the desired 90%. The upside is that changes to the refinery are minimal. In case 2 I have a significant decrease in the energy produced by the turbine but I have 90% CO2 capture. In case 3 I exploit the excess heat of other processes and keep the work of the gas turbine intact. Of course, case three is only possible if extra heat is available from other processes or is produced on purpose.

Andersson, Franck and Berntsson present a techno-economic study of a MEA carbon capture facility deployed in an oil refinery for six different setups. The purpose of their work is to calculate the cost of the different systems as the temperature of the stripper reboiler varies.

So in the paper “Techno-economic analysis of excess heat driven post-combustion CCS at an oil refinery” to minimize costs, they will study the energy flows available as waste in the refinery and then evaluate the benefits of reuse in the stripper. The study focuses on three possible cases of the systems: of the four chimneys present, in the first case only the chimney that emits more CO2 is studied, in the second case the two chimneys that have a higher concentration of CO2 are studied and finally all the chimneys are studied. From previous studies, waste heat is present in the refinery which can be exploited by building a heat collecting system consisting of a trunk pipeline through therefinery with branches out to the different process areas. On the other hand, when the heat is no longer sufficient to power the reboiler, a heat pump is used to cover this demand. The heat pump uses low-grade excess

Figure 3.5: Process diagram of Case 2 of the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through process simulation". [21]

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heat which comes from a pipe system like the one mentioned above. If there is enough heat available at 80 °C, a mechanical vapor recompression pump is used; in the other cases a vapor compression pump is used where butane is the working media.

Table 3.2: Summary of the results of the three case studied in the paper "Study on the integration of fluid catalytic cracking unit in refinery with solvent-based carbon capture through process simulation".

Unit Ref Case Case 1 Case 2 Case 3 Flue gas flow rate kg/s 30.300 30.300 30.300 30.300 Flue gas CO2 content mol% 12.780 12.780 12.780 12.780 Solvent MEA content wt% 32.500 32.500 32.500 32.500

Capture level % 90.000 78.021 90.049 90.000

CO2 captured kg/s 3.490 3.030 3.490 3.490

L/G ratio kg/kg 2.492 1.989 2.492 2.492

Lean loading mol CO2/mol MEA 0.300 0.300 0.300 0.300 Rich loading mol CO2/mol MEA 0.500 0.515 0.497 0.497

WHSG MW 9.960 – – –

Electric power MW 8.080 8.080 4.920 8.080

Steam network energy MW 0.000 0.000 0.000 3.150 Stripper heat duty MW 14.671 11.533 14.671 14.671 Specific duty GJ/ton CO2 4.200 3.803 4.200 4.200

Make up water kg/s 3.037 3.068 3.037 3.037

The results of the work are shown in the Table 3.3.

As the policies say that you are only responsible for the CO2 produced on the site, the CO2

avoided globally will not be taken into consideration. The capital cost per tonne of CO2 is highly similar between the different cases, except for the 90 °C case (due to high cost of VC heat pump), where all flue gas is treated. For the case at 120 °C, the costs are almost the same in all the situations considered. Only in the case in which the treated gases come from a single chimney, it is possible to avoid the purchase of a heat pump that allows you to save money. The cost of treating all gases in the case of a reboiler temperature of 90 °C is not cost-effective.

Table 3.3: CAPEX and OPEX cost breakdown for the anylys of the paper “Techno-economic analysis of excess heat driven post-combustion CCS at an oil refinery”.

90 °C

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