# Optimization of the Energy Consumption of a Carbon Capture and Sequestration Related Carbon Dioxide Compression Processes

^{*}

## Abstract

**:**

_{2}emissions can be mitigated using carbon capture and sequestration (CCS). In turn, understanding how CCS affects the efficient recovery of energy from fossil fuel reserves in different parts of the world requires data on how the performance of each part of a particular CCS scheme is affected by both technology specific parameters and location specific parameters, such as ambient temperature. This paper presents a study into how the energy consumption of an important element of all CCS schemes, the CO

_{2}compression process, varies with compressor design, CO

_{2}pipeline pressure, and cooling temperature. Post-combustion, pre-combustion, and oxyfuel capture scenarios are each considered. A range of optimization algorithms are used to ensure a consistent approach to optimization. The results show that energy consumption is minimized by compressor designs with multiple impellers per stage and carefully optimized stage pressure ratios. The results also form a performance map illustrating the energy consumption for CO

_{2}compression processes that can be used in further study work and, in particular, CCS system models developed to study performance variation with ambient temperature.

## 1. Introduction

_{2}emissions, but its wide spread adoption is held-back by uncertainty over the energy consumption and cost impact implied by the additional infrastructure required because of this, the optimization of CCS processes to reduce their energy and cost impacts represents an important field of study.

_{2}captured (kWh

_{e}/tCO

_{2}), whereas estimates of the energy consumption for CO

_{2}compression lie typically in the range 80–120 kWh

_{e}/tCO

_{2}[2]. CO

_{2}compression is also a mature technology, with conventional multi-stage centrifugal CO

_{2}compressor designs widely used in the fertilizer and petroleum industries [3]. Suppliers of large-capacity multi-stage centrifugal CO

_{2}compressors include Dresser-Rand, General Electric (GE), and MAN Turbo and Diesel (MAN). Common industrial applications include enhanced oil recovery (EOR), fertilizer production, and CCS. MAN, for example, has operating references including an 8-stage compressor used for EOR with a discharge pressure of 187 bara and a capacity of 36 kg/s; a 10-stage compressor used in fertilized production with a discharge pressure of 200 bara and a capacity of 13 kg/s; and offers designs for capacities of up to 110 kg/h [4]. Given that a 600 MW natural gas combined cycle power plant with 90% CO

_{2}capture requires a CO

_{2}compression capacity of approximately 56 kg/h [5], the scale-up of current industrial designs to CCS applications is generally considered unproblematic. As a result, fewer studies have been made into the optimization of the CO

_{2}compression process than have been made for the capture processes.

_{2}compression in the context of a particular CCS capture technology. Romeo et al. [6] and Luo et al. [7] studied the optimum design for CO

_{2}compression in the context of heat recovery into a steam cycle. Posch et al. [8] and Font-Palma et al. [9] studied the optimization of the oxyfuel flue gas purification process where the early parts of the CO

_{2}compression process are integrated with the CO

_{2}separation process. Other studies have looked at optimization from the perspective of comparing conventional compression approaches to newer unconventional approaches. Alabdulkarem et al. [10] studied the potential benefit of liquefaction and pumping. Harkina et al. [11] and Luo et al. [7] studied the benefits of using shockwave type compression. Pei et al. [12] studied the benefits of heat recovery using ORC. A small number of studies such as Calado [13] and Jackson et al. [14] have considered the optimum number of CO

_{2}compression stages, but these have had a limited focus and do not apply their conclusions to the full set of operating parameters that could be expected in a range of typical CCS scenarios. Overall, most CO

_{2}compressor optimization studies are limited to one specific CCS scenario and focus on the capture process rather than the compression process in itself. Many do not optimize stage pressure ratios or the number of compression stages. None have been identified that consider optimization of compressor design for a wide set of operating cases.

_{2}compressor performance data that is based on the optimum number of compressor stages and stage pressure ratios for a wide range of operating cases. The specific intention is that this data can be used to support the development of a system model capable of comparing energy consumption of a wide variety of CCS scenarios.

## 2. Method

_{2}compressor design parameters; the operating parameters for the three principle CO

_{2}capture alternatives: Post-combustion, pre-combustion and oxyfuel; and the appropriate range of cooling temperatures and discharge pressures to be studied. Optimization of the compressor model for each set of parameters was carried out using algorithms available in MATLAB. Each of these elements of the study method are set out in Nomenclature.

#### 2.1. Compressor Modeling

_{2}compressor designs can either be integrally geared type, with one impeller and one gas cooler per stage; or barrel type, with multiple impellers and one gas cooler per stage [3]. In both designs the pressure ratio per impeller is limited to around 2 for CO

_{2}service and because of this, it is common to model CO

_{2}compressors with a pressure ratio per stage limit, $Pr$ < 2 [6]. In this study, the cases following this convention are called the constrained cases. In a barrel type compressor design it is common to use several impellers per stage, and with an integrally geared design this is also possible, although less common. When multiple impellers are used per stage, $Pr$ can be greater than 2, and in some studies, unconventional compressor designs with a $Pr$ of up to 10 have been investigated [7,11], although these compressor designs are still under development. In this study, all cases where $Pr$ > 2 are referred to as unconstrained cases.

_{2}compressor design with $n$ stages, where $Pi{n}_{1}$ is the compressor feed pressure and $Pou{t}_{n}$ is the compressor discharge pressure. Each stage is assigned a sequential number, $i=\left\{1,\dots ,n\right\}$. $Pi{n}_{i}$ is the inlet pressure for stage i; $P{r}_{i}$ is the pressure ratio for stage i; and $Pou{t}_{i}$ is the outlet pressure for stage i. The gas cooler pressure drop, ΔP, and the gas cooler outlet temperature, To, are both equal for all stages. In all cases, $\Delta P$ is fixed at 0.5 bar and To is studied over a range.

_{c}, which was calculated in the model as follows:

_{2}stream is dry and ${x}_{i}$ is therefore fixed. In the other cases, the CO

_{2}stream is saturated with water below 30 bar(a) and, therefore, the water content and ${x}_{i}$ varies with temperature and pressure. In these cases, ${x}_{i}=f\left(To,Pi{n}_{i,}\right)$ was also calculated using TREND.

_{2}mixture,${P}_{CR}$. The final stage of the compressor represents a pump when the CO

_{2}stream is in the liquid phase.

#### 2.2. Study Parameters

_{2}capture scenarios were modeled: Pre-combustion capture, post-combustion capture, and oxyfuel. In all of the three scenarios the compressor discharge pressure, $Pn$, is studied in the range 90 bar(a) to 180 bar(a). Each compressor aftercooler has an outlet temperature, $To$, which is studied in the range 288 K to 323 K. The scenario specific parameters for each of the three capture cases are described below and summarized at the end of this section in Table 2.

#### 2.2.1. Post-Combustion Capture

_{2}at atmospheric pressure that is saturated with water. Typically, CO

_{2}streams that are to be transported in a pipeline are dried part-way through the compression process to take advantage of water drop-out in the early stages of compression [2]. In this study the break point for dehydration was taken as 30 bar(a): All stages below 30 bar(a) were modeled with a feed stream that was saturated with water; all above were modeled as dry. The dry stream composition used in this study, 99.99 mole% CO

_{2}and 0.01 mole% nitrogen, was based on an assessment of data published by DNV [17] and the TRENDS project [18].

#### 2.2.2. Pre-Combustion Capture

_{2}originating from steam-methane reformer that is captured using an MDEA solvent. As in the post combustion cases, the feed stream will be at atmospheric pressure and saturated with water below 30 bar(a). The dry composition, 99.5 mole% CO

_{2}and 0.5 mole% methane, was, again, based on an assessment of data published by DNV [17] and the TRENDS project [18].

#### 2.2.3. Oxyfuel Capture

_{2}compressor are set to optimize the performance of the purification process. This type of optimization falls outside of the scope of this study and, therefore, the feed pressure for the CO

_{2}compression process studied here is taken as the highest product stream pressure resulting from the oxyfuel purification process, i.e., de-coupled from the optimization of the purification process. The CO

_{2}stream from the purification process is dry, the main impurities being N

_{2}, O

_{2}, and argon [17]. The level of these impurities is not limited by technical barriers [9] and technologies for high and low purity has been investigated [8,15]. In this study, the highest pressure of the CO

_{2}feed stream leaving the purification unit is taken as 16.5 bar(a) and its composition 96.16 mole% CO

_{2}, 2.45 mole% nitrogen, 0.96 mole% argon, and 0.43 mole% oxygen based on Posch et al. [8].

#### 2.3. Optimization of Compressor Energy Consumption

#### 2.3.1. Variables, Initial Guesses, and Constraints

#### 2.3.2. Objective Function

#### 2.4. Identifying the Optimum Number of Compression Stages

## 3. Results and Discussion

#### 3.1. The Benefits of Optimization

#### 3.2. Consistency Checking and Optimum Stages

#### 3.3. Variation of Energy Consumption with Cooling Temperature and Pressure

_{2}capture scenarios. The optimum number of compression stages was overlaid.

#### 3.4. Constrained vs. Unconstrained Cases

## 4. Discussion

_{2}compression cases varies in the range 292 kJ/kg CO

_{2}to 425 kJ/kg CO

_{2}. This compares well with the range of 80–120 kWh

_{e}/tCO

_{2}(equal to 288–432 kJ/kgCO

_{2}) reported by Jordal et al. [2].

_{2}compressor with a 14 MPa discharge pressure and 50 °C cooling (representing warm climates) to be 104.6 kWh per ton of CO

_{2}(= 380 kJ/kg of CO

_{2}), which compares well with 390 kJ/kg of CO

_{2}found in this study, Figure 7a. Alhajaj et al. [22] also reported an energy consumption of 82.4 kWh/ton of CO

_{2}(= 297 kJ/kg of CO

_{2}) for the same compressor with a 20 °C cooling temperature (representing colder climates), which also compares well with 311 kJ/kg of CO

_{2}found in this study from Figure 7a.

_{2}stream will condense in seven stages and, hence, the overall minimum stages is eight. When $To>293$ K, nine stages are required. Since the energy consumption for the eight and nine stage cases also falls within 2% of the overall minimum, as illustrated in Figure 4a and Figure 5a, they also satisfy the criteria for the optimum number of compression stages used in this study. This means that the optimum number of stages presented in Figure 7a and Figure 8a is dictated by the practical minimum number of stages for these cases.

## 5. Conclusions

## Nomenclature

$Hi{n}_{i}$ | Enthalpy at the inlet to stage i |

$Hout{S}_{i}$ | Enthalpy at the outlet if stage i (isentropic basis) |

n | Number of compressor stages |

${P}_{CR}$ | Crycondenbar pressure |

${P}_{DP}$ | Dew point pressure |

$Pi{n}_{i}$ | Inlet pressure pressure for stage i |

$Pou{t}_{i}$ | Outlet pressure pressure for stage i |

$P{r}_{i}$ | Pressure ratio for stage i |

∆P | Aftercooler pressure drop |

${S}_{i}$ | Entropy for stage i |

To | Aftercooler outlet temperature |

W_{c} | Energy used in compression |

${x}_{i}$ | Gas composition stage i |

$\eta $ | Isentropic efficiency |

## Supplementary Materials

**a**) constrained cases and (

**b**) unconstrained cases. Figure S2: Pre-combustion capture cases, compression energy consumption with fixed (un-optimized) stage pressure ratios, (

**a**) constrained cases and (

**b**) unconstrained cases. Figure S3: Oxy-combustion capture cases, compression energy consumption with fixed (un-optimized) stage pressure ratios, (

**a**) constrained cases and (

**b**) unconstrained cases. All data from Figure 7, Figure 8, Figure 9 and Figures S1–S3 in CSV .txt format.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 3.**Energy consumption for a 10 stage CO

_{2}compressor, post-combustion capture case, ${P}_{n}$ $=90$ bar(a) and $To=288$ K with (

**a**) constant $Pr$ and (

**b**) optimized $Pr$.

**Figure 4.**Relative energy consumption vs. stages for post-combustion capture, (

**a**) constrained cases and (

**b**) unconstrained cases (right).

**Figure 5.**Relative energy consumption vs. stages for pre-combustion capture, (

**a**) constrained cases and (

**b**) unconstrained cases.

**Figure 6.**Relative energy consumption vs. compressor stage, oxyfuel cases, (

**a**) constrained cases and (

**b**) unconstrained cases.

**Figure 7.**Post-combustion capture cases, optimized compression energy consumption and number of stages, (

**a**) constrained cases and (

**b**) unconstrained cases.

**Figure 8.**Pre-combustion capture cases, optimized compression energy consumption and number of stages, (

**a**) constrained cases and (

**b**) unconstrained cases.

**Figure 9.**Oxyfuel cases, optimized compression energy consumption and number of stages, (

**a**) constrained cases and (

**b**) unconstrained cases.

**Figure 10.**Reduction in energy consumption (%) for the unconstrained cases relative to the constrained cases with reduction in the number of compression stages overlaid, (

**a**) post-combustion cases, (

**b**) pre-combustion cases.

Parameter | Constrained Cases | Unconstrained Cases |
---|---|---|

Pressure Ratio, $Pr$ | <2 | <10 |

Efficiency, $\eta $ | 85% | |

Stage pressure drop, $\Delta P$ | 0.5 bar | |

Final stage inlet pressure, $Pi{n}_{n}$ | Minimum of ${P}_{DP}$ + 5 bar and, ${P}_{CR}$ + 5 bar |

Parameter | Post | Pre | Oxyfuel |
---|---|---|---|

Cooling temperature,$To$ | 288 K to 323 K | 288 K to 323 K | 288 K to 323 K |

Discharge pressure, $Pn$ | 90 bar(a) to 180 bar(a) | 90 bar(a) to 180 bar(a) | 90 bar(a) to 180 bar(a) |

Inlet pressure,$Pi{n}_{1}$ | 1.01 bar(a) | 1.01 bar(a) | 16.5 bar(a) |

Dry stream composition | CO_{2} 99.99 mole% N _{2} 0.01 mole% | CO_{2} 99.5 mole% CH _{4} 0.5 mole% | CO_{2} 96.16 mole% N _{2} 2.45 mole% Ar 0.96 mole% O _{2} 0.43 mole% |

Parameter | Post const. | Post u.con. | Pre const. | Pre u.con. | Oxy const. | Oxy u.con. |
---|---|---|---|---|---|---|

Energy, W_{c} [kJ/kg(CO_{2})] | 292 to 406 | 312 to 425 | 316 to 431 | 294 to 140 | 87 to 150 | 87 to 150 |

Stages, n (-) | 8 to 9 | 6 | 8 to 9 | 6 | 4 to 5 | 4 to 6 |

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**MDPI and ACS Style**

Jackson, S.; Brodal, E. Optimization of the Energy Consumption of a Carbon Capture and Sequestration Related Carbon Dioxide Compression Processes. *Energies* **2019**, *12*, 1603.
https://doi.org/10.3390/en12091603

**AMA Style**

Jackson S, Brodal E. Optimization of the Energy Consumption of a Carbon Capture and Sequestration Related Carbon Dioxide Compression Processes. *Energies*. 2019; 12(9):1603.
https://doi.org/10.3390/en12091603

**Chicago/Turabian Style**

Jackson, Steven, and Eivind Brodal. 2019. "Optimization of the Energy Consumption of a Carbon Capture and Sequestration Related Carbon Dioxide Compression Processes" *Energies* 12, no. 9: 1603.
https://doi.org/10.3390/en12091603